A common type of expansion card is a sound card. Most IBM PC systems have only a simple speaker for sound output. While this is fine for simple beeps and tones, it is not well suited to complex sounds like music or speech. Further, the computer's |CPU| must constantly control the speaker to get anything other than a single tone. Tandy computers added special circuitry to enhance the sound capabilities of their systems, but this was not available to other vendors.
Sound expansion cards allow the computer to ~synthesize~ music with several ~voices~. They also handle much of the work needed to generate these sounds, freeing the CPU to do other activities. Other cards go beyond this, adding ~digital channels~. These can very realistically reproduce almost any sound, including speech. Some cards support stereo output and others also add a MIDI port. The higher end cards add the ability to control CD-ROM drives. This means they can handle music CDs as well as their other channels. The different signals are all ~mixed~ and amplified by the card's circuitry before being output to speakers.
