Just posting some findings using headphones via the onboard 3.5mm audio jack via Raspbmc.
It seems that distortion is present at any audio level in high frequencies using the following test files:
MP4 video with 192kbs AAC audio
MVK video with 384kbs Dolby Digital
I'm not sure if this is being caused by any of the following at present:
Bad screening with the audio jack/board being placed close to the monitor/screen.
5v power issue where the board is running at extreme minimum voltage input.
Codec issue which is causing peeks/clipping (however its present at all volumes).
Any audiofiles out there done any testing of analog audio quality in debian/arch finding the same???
I'm currently powering my Pi via a Motorola lapdock - and will be doing futher testing with 5v 1amp charger today to see if its a voltage thing and will feedback.
Any guidance/knowledge in the meantime would be most apreciated
Update : Just fed the board with 5volts @ 1 amp - same results - Its deffo not power related