I found a nice way to use the audio tag, Audio for everybody. It tries first to use the html5 player, then wmp, then flash, then an image.
Unfortunately, the mp3 format isn't supported on firefox even if it's supported by the os, and it displays an X instead of degrading to the flash player.
There's a way to let firefox ignore the html5 player?
ps: converting all my mp3s to ogg it's not an option