The DHTML runtime doesn't support mp3 audio playback. Check the development guide section on media resources: http://www.openlaszlo.org/lps4.9/docs/developers/media-resources.html#d0e39353 "Mp3s are not natively supported by browsers, so applications that compile to DHTML should make provisions, as described above."
If you wonder if a feature is supported, check the LzSprite.capabilities list. For the DHTML runtime, LzSprite.capabilities.audio should be 'false'. - Raju
