My PC has a build-in VT1828S chip. The connection between PC and monitor (which has built-in speakers) is through an HDMI cable.
Audio preferences shows 2 devices for audio output: analog stereo and RS880 Audio Device [Radeon HD 4200] digital stereo (HDMI).
When I plug a headphone in the rear jack and I choose the former device, everything works.
When I choose the latter device, instead, no sound is heard, and video playback is too fast. It is as if video frames want to synchronise with the audio timing, but the audio timing information is wrong.
HDMI audio worked perfectly with Ubuntu 11.04, but stopped after 11.10 upgrade.