New motherboards have integrated sound chipset (some even support 7.1 surround sound) which is good enough for most folks, however, some people who doesn't satisfy with average sound quality should look for a better alternative. If you need a super quality sound then an after market sound card is required. If you are just a simple computer user or gamer who doesn't require super quality sound, going with on-board sound is probably a good choice.
The main advantages of having a sound card in comparison to on-board sound are; better sound quality (higher S/N ratio and higher bit rates) and features such as EAX and optical ports.
Main factors affecting sound quality are as follow:
Signal to Noise Ratio (SNR):
SNR typically measured in dB, shows the ratio of received signal level that has corrupted by noise.
Total Harmonic Distortion (THD):
THD measures in percent (%), is amount of harmonic distortion generated by sound amplifier. Harmonic distortion occurs when the amplifier creates frequency content that is not present in the input signal. Lesser THD allows the components in a loudspeaker, amplifier or microphone or other equipment to produce a more accurate reproduction. A THD rating less than 1% is considered to be in high-fidelity and inaudible to the human ear.
A/D (Analogue to Digital) and D/A (Digital to Analogue) conversion resolution and accuracy:
Expressed in bits, many sound cards support 24-bit and even 32-bit conversion resolution at 192 KHz sample rate.
Frequencies are measured in Hertz (Hz). Audio frequencies normally ranging from 20 Hz 20 KHz which is a limit of human hearing, although these limits are not definite. Most of sound cards can support wider range of frequencies, specially low frequencies which produce better base sound than integrated sound chipsets.