Been dealing with this issue since the Black update as well. in my testing of my Lumia 920, I've found the volume to work just fine for apps like games. Volume goes up and down as I see fit. HOWEVER, when it comes to music, podcasts, anything to deal with audio playback (Spotify, XBox Music, Bringcast), the music track or podcast will be stuck at whatever the volume is set at when it begins. It will not change. pausing and continuing a track will not adjust it. restarting the track/podcast AFTER changing the volume will have it at the new volume level but again, cannot be adjusted. doesn't need a reboot or restarting of the app to get the volume level to readjust to the new level.
for those still paying attention to this thread, see if the same happens to you. it would be interesting to know if this particular situation occurs across the board or if my resolutions are specific to my phone only.
thanks
EDIT: I thought I tried this but did it again anyways: rebooted my Lumia 920 and now the volume controls work on music and podcasts. Let's see how long this lasts...