What is an MP3?
MPEG layer 3 is a type of audio codec where processed by significant compression from the original audio source with very little loss in sound quality. The compression up to 12:1 produces a very little degradation. Tighter compression can be achieved, but it will effect in sound degradation results.

A standard is to encode at anywhere from a quality 128 up to 320 kbit/s (near cd quality result). Many people claim that low-rate MP3 files sound better than Real Audio files with similar bit rates. The higher the bitrate(quality), the larger the file.

If the music is going to be streamed from the page (not downloaded first) then the standard is 128 for high speed connections (DSL, Cable) and 24 to 32 for dial-up connections.