if you want them to play through the browser remember there are going to be a few who can't hear mp3. Its the same for real audio and flash movie. If you want it to be universal and reach the last x% (there are more than you think) consider WAV files. Same goes for moving words and some of the fancy IE6/Netscape 7 finery.
Some 'catters have WebTV browsers and they seem to have difficulty with Java. I try my pages on IE5.5 / Netscape 4.5 and that does not cover enough old browsers. And then there is the Mac collection............ Its anarchy out there I tell you.
I usually prevail on the goodwill of friends and 'catters to have a look for me. Its amazing what you miss.