I didn't think that it was possible, either (there was actually a discussion of that here a while back). I guess that to be certain, though, I'd have to dissect a DVI-HDMI cable. The question in my mind is whether some of the HDMI conductors are abandoned at the DVI connector or whether all of them find a home in the DVI pins. If they all get carried over to the DVI connector, then the audio signal could be passed through the 990 and on to the TV.

A little poking around in google may give us some hints. HDMI has 19 pins (the type B connector that would be needed to handle higher resolutions will use 29 pins, but I don't think it's actually in use yet since it's only needed to support resolutions above 1080p). It is also pin-compatible with single-link DVI (meaning that if we connect the right pins of the HDMI connector to the right pins of the single-link DVI connector, we get video - and type B HDMI is pin-compatible with dual-link DVI). The 990's DVI video switching supports a dual-link DVI connection, I believe (at least it appears to have conductors in place for the dual-link portion of the connector). Dual-link has 24 pins, five more than the total pin complement of HDMI. I guess that it would be possible for two identically-made cables that used DVI-D dual-link connectors and kicked the HDMI audio signals to the same extra pins provided by dual-link, which would then allow the audio signal to pass through the 990's DVI switching.