cwirick, welcome to the Seven Forums.
I would start with checking you have the Nvidia HDMI audio driver installed, NVIDIA DRIVERS 1.00.00.59 WHQL
Also is the source 5.1/surround or stereo only? If it is stereo then an upmix somewhere along the line is required to get surround sound.
Also found this thread here, Acer Aspire Revo Owners Thread - AVForums.com
No idea if the answer is there, but seeing as it is 34 pages long I would hope someone has brought it up.