What is it?
A sound card - otherwise known as a sound board, audio card, or audio adapter - is an internal device that enables your computer to output sound through speakers, record sound with a microphone, and manipulate sound stored in files or on disk. If your computer monitor doesn't have built-in speakers, you will need separate external speakers in order to produce audio or you can use headphones. 

How do I go about buying one? 
Sound Cards
Standards: Although there is no official sound card standard, Creative Labs' popular Sound Blaster line has become a de facto standard. A Sound Blaster-compatible card should run on virtually any system and with any application. 

A card that requires special drivers for Sound Blaster compatibility may be okay, but can cause problems and take up additional memory that could be applied to your applications. It's best to purchase a fully Sound Blaster-compatible card. 

Types: Sound cards employ two common methods of creating sound: FM (frequency modulation) synthesis or wavetable synthesis. 

FM synthesis creates sound by mimicking musical instruments according to preset formulas. This is more common and less expensive and adequate for the common consumer. However, it doesn't sound as good as wavetable synthesis, which is preferred by gamers and enthusiasts. 

Wavetable synthesis relies on recordings of actual instruments to create sound, rendering more accurate sound, but increasing the cost of the card. If you're serious about music and gaming, a wavetable synthesizer is a worthwhile investment. You can either purchase a new sound card or add a daughter card, such as Creative Labs' WaveBlaster, which attaches to your existing FM synthesis SoundBlaster card and performs wavetable synthesis. 

Slot interface: Sound cards connect via the following interfaces: 

PCI: If you have a Pentium system, you probably have a PC local bus called PCI (Peripheral Component Interconnect.) If so, get a PCI card - it's faster. Furthermore, the speedy 32-bit PCI is becoming the accepted norm in the world of PCs, and even Macs, since PowerMacs, employ a PCI bus. 

ISA: The ISA (Industry Standard Architecture) bus is slowly being replaced by PCI. Because ISA is only eight-bit or 16-bit, it has its limitations. If you have an older computer (IBM PC 486 or lower), chances are it's an ISA bus system, so look for an ISA card. Gamers should avoid ISA cards, which can slow you down. 

PC Card (PCMCIA): This is the interface for notebook computers. Purchase a card with a PC Card interface only if you plan on using it with your portable PC. 

3D Sound: Hot with gamers, this technology simulates the effect of deep, detailed sound coming from various directions. Even in a cramped space with small speakers, 3D achieves the aural "illusion" of a vast landscape and multiple sources. This effect is particularly impressive with 3- or 4-piece speaker sets. If you invest in a 3D sound card, all of your multimedia applications will benefit from 3D's lush spaciousness. 

Number of Simultaneous Voices: This specification indicates the number of separate sounds that the card can process and play at once. A 64-voice card theoretically reproduces the sounds of 64 instruments, simulating an orchestra. A higher number of simultaneous voices generates sound with greater richness, depth, and realism. Musicians, gamers, and anyone who creates sound files benefit from a higher number of simultaneous voices. 

MIDI: MIDI (Musical Instrument Digital Interface) compatibility allows the card to play back MIDI files, which are far smaller, easier to manage, and quicker to download than other sound files, such as WAV. There are countless sites on the Internet that contain downloadable MIDI files for listening enjoyment and creating MIDI files can be fun for music lovers and valuable to music professionals. 

If you're a composer, a number of software programs are available for composing and editing music that conforms to the MIDI standard. For instance, a music program can translate what you play on a keyboard into a written score. A MIDI Connector is necessary for hooking up your computer to other MIDI devices, such as musical instruments and composition and sequencing tools. 

Digital Signal Processors (DSP): DSPs add intelligence so the sound card can perform its own processing, which frees the computer from demanding tasks, such as file compression, and enables various effects, such as surround-sound and reverb. In short, DSPs turn a sound card into a multi-purpose device. Don't worry about this unless you're an enthusiast, business professional, musician, MIDI-maniac, or you want to produce your own sound files and create sound effects. 

Speakers
Does your little monitor speaker sound as cheesy as a drive-through intercom? Here are the major considerations when shopping for sweet PC sound. 

Watts: This figure represents the amount of amplification driving the speaker, usually expressed as watts per channel or "RMS." The more watts, the more power, which enables more volume without distortion. Music and gaming especially benefit from vigorous speakers. 

Power supply: Sound cards lack the power to drive speakers, so most speakers incorporate their own power supply. Notebook users need speakers that can operate with batteries, since electrical outlets won't always be within reach. Moreover, it's not a bad idea to invest in some rechargeable batteries, which save money in the long run. And since batteries expire quickly, home users should buy speakers that come with an AC adapter, or at least accept a separately sold AC adapter, or your trash bin will be piled high with batteries. 

Sleep: This function powers down your speakers if idle for a certain period of time, conserving power - important if you're using batteries, crucial if you're a notebook user. 

Subwoofer: Many speaker sets include a subwoofer, which renders rumbling, high fidelity bass. If you already own a pair of satellite speakers, you may still obtain a separate subwoofer to beef up your sound. Though a subwoofer costs extra, it significantly improves the range, depth, and realism of sound, especially impressive for music and action games. If you care about sound quality, a subwoofer adds the essential lows to the highs and mids of satellite speakers. Many subwoofer designs fit even in confined spaces or under your desk. 

Dynamic bass boost: When activated, this function amplifies the bass. Bass boost is good to look for if you can't afford a subwoofer system but still desire decent bass. Also, since bass becomes inaudible at low volumes, bass boost comes in handy when you're playing music softly, adding bottom and balance to music turned down low. In addition to bass boot, a treble or tone control helps you maintain the sound you like at different volumes and from different audio sources. 

Number of pieces: A 2-piece speaker set, which consists of two satellite speakers and no subwoofer, is adequate for basic multimedia. However, low-watt speakers may sound tinny, especially if they lack bass boost. For notebook use, a tiny pair of speakers transport more easily, but be sure they can run on batteries. 

Gamers will want a 3-piece system, which consists of two satellite speakers and a bass-rich subwoofer. A compact 3-piece design fits into surprisingly cramped quarters while still providing the deep bass vital to multimedia excitement. 4-speaker rigs are actually a misnomer, because the "4" refers to the number of satellites; there's still a fifth box in the form of a subwoofer. DirecSound3D and Creative Labs EAX API sounds best with this last option, though you'll need a sound card with two discrete audio outputs. 

Audio enthusiasts will appreciate a multiple speaker set - the sound, especially with 3D, will knock your socks off. In fact, top-of-the-line, multiple PC speaker sets often sound as good as high-end home stereo systems, which cost several times more. Therefore, if you use your PC to play music as well as games, a great set of PC speakers can also serve as a good home stereo. In other words, investing in high-end PC speakers yields two birds for one stone: blow-away-quality speakers for hot video games and a killer stereo for favorite CDs and DVDs. 

Headphones: If good speakers cost too much, or if you want private audio, headphones offer another solution. More expensive headphones give you richer sound, volume controls, more comfortable design, and large ear cushions that reduce noise bleed so that people around you won't hear your music.