I found the answer to my question in
this review. Here's an excerpt from the review:
Strangely enough AOpen equips the Mini PC with a Realtek ALC850 AC’97 codec which is an 8-channel codec when the Mini PC only has a single stereo output. A two-channel high-definition audio codec or Analog Devices AC’97 codec would have been a better choice.
Since I'm not very familiar with PC Audio, I don't really understand this explanation, but it seems to indicate that AOpen's implementation is to blame.