Now that I've fixed the issue, the problem is obvious. I have a Gigabyte motherboard and I use the SPDIF output to take the sound via optical cable into my surround sound receiver. In the video setup, I had the default player set to mplayer with the command:
mplayer -ao alsa:device=spdif -fs -zoom -quiet -vo xv %s
This directed sound to the SPDIF output. But, when using MythStream, no sound came out when watching things such as the Apple movie trailers (MythStream doesn't let you specify settings for mplayer like MythVideo does). Thus the tinkering began. What I needed to do is configure mplayer to always send sound to SPDIF. You can do this by specifying settings in ~/.mplayer/config
Now, everything is good. Mplayer sound always goes via SPDIF, whether I'm playing it through MythTV (MythVideo) or not.
Note that before I arrived at this solution I'd tried other settings in the mplayer config. When I had the settings wrong I saw the following when running mplayer from the command line:
Forced audio codec: hwac3
Cannot find codec for audio format 0x73627276.
Audio: no sound
This was occurring because I'd been googling for solutions and was lead to try values such as:
in the mplayer config file.