I wanna try this... sounds really cool.
Just found a simple workaround to enable local mp3 support without negative side-effects on existing functionality:
in the exported project's media subfolder, batch-convert all m4a to mp3 with the software of your choice, or for better quality, start with the uncompressed sound files (wav) if you have them.
Then open c2runtime.js with a text editor, search for "m4a" and replace it with "mp3".
The project will still have sound enabled in all currently supported browsers, using mp3 instead of m4a for Safari and IE 9.
Unfortunately, it doesn't enable sound on mobile webkit - be it Apple's or Google's version - so there's no point to do this as of now, because it will only slightly increase the project's size due to mp3 compression being generally weaker than m4a and additionally it might put you at risk with the format's license restriction for games.
Probably the method used to create/initialize the audio object in c2runtime.js doesn't play well with those mobile webkit flavors (in fact I even think this was already discussed here or in another thread) - could you use a method that outputs the same html code as in the w3schools html5 sound example I posted, which does work correctly on all mainstream html5 browsers, including the one in Android 2.3?
This should solve the sound issue on iOS and Android, for the latter provided that it supports ogg or m4a (who knows, anyone can change, even Google!..) in an upcoming version; in the meantime, my mp3 workaround would give Android in-game sound to the reckless <img src="smileys/smiley2.gif" border="0" align="middle" />.