Man, I hate registering just to post a one time reply on a zombie post, to a forum I'll never visit again, but its high up there in the Google search, so whatever.
I recently came across a stange issue where my headphones were working but then they all of a sudden quit. The headphones were connected, the output was set to the headphones and the media was enabled, but there's no sound at all.
None of the fixes as shown above or anywhere else for that matter worked, (making sure media was set to a proper level, resetting the headphones, etc..) and the headphones worked fine on my wife's Ipod so the headphones were not at fault; I could prove it.
The issue turned out to be that the headphones were using AAC for the codec, which Android kinda supports, but I guess my luck ran out.
The solution was to first make sure the Bluetooth headphones were connected and set as the output device, then enable developer mode in the settings (in my case Settings > About Phone and tap the version number seven times to enable it), and then under Settings > Developer Options there is an option to disable HD Audio. Sliding HD audio to disabled allowed me to change the codec settings from AAC to SBC on the option below the HD Audio Slider (also in Settings). Failing to first disable HD audio first may leave the SBC option greyed out. After that, all was good. That's it. No other changes like bitrate and such were necessary.
So frustrating to not have anyone else seem to have this problem, and I figured I couldn't be the only one, so I hope this helps someone who feels this stuck as I did and has tried all the regular options.