Sorry, couldn't find any place to post this. It seems that there is, at least on my phone, a semi-reproducible bug that stops the volume control from working.
Steps to reproduce (from clean phone, ie. just freshly booted, no remnant IE tabs):
1. Turn on phone.
2. Open music+videos (Xbox music) app. Play a song that is locally stored on the phone.
3. While the music is still playing, open up an Internet Explorer tab and navigate to youtube (m.youtube.com).
4. Play an HD movie file. I played Peter Jackson's "The Hobbit" preview #9. Your previously playing music will stop playing.
5. After a few seconds, stop the youtube video playback by hitting the Windows button.
6. Go back to the music+videos app. Play a song that is locally stored on the phone.
On times that the bug shows up, the music plays at a fixed volume. Using the volume rocker will cause the volume display to increase and decrease, but the actual volume will not change unless you change the volume to 00/30, at which point the sound will mute. If the bug doesn't show up, repeat steps 3 through 6 until the issue shows up. You don't need to reboot in between attempts.
Interestingly, volume control still works if you go back to the youtube video. The issue shows up when using the loudspeaker as well as when using headphones. If nobody else can reproduce it, we can start trying to figure out if it's certain music file types or something that cause it. Diagnostic information below:
Model: NOKIA Lumia 920
OS Version: 8.0.9903.10
Firmware revision number: 1232.2109.1242.1001
Hardware revision number: 126.96.36.199
Radio software version: 1.0.202041.3
Chip SOC version: 8960