IF you have to do that (say a joke site), unless you want to make the viewer download a plug-in, you basically need to stick to something the browser is already prepared to play. Needing smallish sizes, that arguably is MIDI, MP3 ... or maybe one of the formats handled by an installed media player (WMP, Real, Quicktime, etc.). Depending on how you want to handle it, you can embed those formats. For size likely MIDI, then AAC/Real/WMA/Ogg, then MP3. Personally I'd go with MIDI or AAC, but it's up to you to figure out your user base. Also check some band sites out and see how they handle things.