Most mp3s are encoded at 128 and 192 kbps, but I was working on a project that used speech recordings, and speech is usually encoded at a lower bitrate than 128. I found every time I played the recording in a flash based mp3 player it played it too fast, making it sound like alvin and the chipmunks.
I went through and made a whole bunch of mp3s at different bitrates using the lame 3.96 encoder to see which were supported and which bitrates were not.
So the basic synopsis is mp3s encoded as 48kbps work and anything greater or equal to 96kbps also works. Everything else is not supported (is support, but sounds like chipmunks).
for another possible explanation of the chipmunk sounds.