You can fix it now: Sample rates of 96000, 44100, 22050 and 11025 samples/second at 8, 16 and 32 bit depth will all work fine. So if you wish to remove the chipmunks from your site you will have to re-encode the offending files using the sample rates given above with one of the constant bit rates. If you are interested there is more information about this bug available.

Adobe will fix it later: This bug will be fixed soon by the release of Flash Player Update 3 which is currently at the Beta 2 stage which will play back any MP3 sampling rate. So, no more chipmunks. Ever. But this will require all your visitors to have downloaded the soon to be released Flash update for this to work, i.e. your visitors must have the update to hear your music correctly.

So you have an option. You can re-encode your music and remove the chipmunks now, or you can wait for the fix to hit the masses.