As Luke said HDMI audio on Radeon hardware has been disabled by default by the upstream kernel developers, as it causes blank screens on a lot of systems. You can re-enable HDMI audio by adding "radeon.audio=1" to your GRUB boot options. There are reports here in Launchpad and on Phoronix that this also fixes the fast video playback problem.
As Luke said HDMI audio on Radeon hardware has been disabled by default by the upstream kernel developers, as it causes blank screens on a lot of systems. You can re-enable HDMI audio by adding "radeon.audio=1" to your GRUB boot options. There are reports here in Launchpad and on Phoronix that this also fixes the fast video playback problem.