While done by an independent developer, the Novell-supported ATI driver is capable of outputting audio over HDMI where as the other open-source driver has no support at this time (the proprietary Catalyst Linux driver also supports HDMI audio). With the ATI Radeon HD 2000 (R600) series and later, there is an integrated audio processor for transmitting audio over a DVI/HDMI connector. Besides needing a newer release of xf86-video-radeonhd driver, a supported version of ALSA is a requirement.
Anyways, submitted on the RadeonHD mailing list this afternoon is a 275-line patch for providing RS690 HDMI audio support. This patch, which adds HDMI audio support for this popular ATI IGP found in HTPCs, was submitted by Christian König, who had originally written the 2,000+ line patch providing the initial audio support, but the RS690 patch was based upon the work of Christiaan van Dijk.
The patch hasn't yet been committed to the xf86-video-radeonhd Git tree, but we could see it there soon.