I have this problem too. Bought the Nexus 7 the first week it came out. Didn't notice any sound problems then. That was originally Jelly Bean 4.0 I believe. Then the system updated to 4.1 and finally to 4.2.2 which I have now. Then I noticed a big sound problem. Low volume both through the built in speaker and the headsets. With headsets, I couldn't hear the music above the ambient room noise. Since the warranty was about to run out in 1 week, I called ASUS. They didn't offer any solutions but they did issue an RMA to send it in for warranty repair. The RMA form required me to agree to let them reset my tablet which would delete all information. I don't have anything critical on my tablet so why not? But I decided I might as well do that myself instead of paying shipping charges to Grapevine, Texas where the warranty facility is located. I went ahead and uninstalled all the apps I added also. Well what do you know...it fixed the sound problem. NOTE: I failed to mention that when I would start the music player, it would have normal volume for the 1st few seconds before it dropped low, not to recover. Since the system restart, the volume is good now. I've been playing it for a few days now and the sound has stayed consistent.
I post this realizing that many of you have tried a system reset and it didn't fix your problem. After reading several comments like that, I wasn't even going to try doing so on mine. But since ASUS was going to do it, I figured I had nothing to lose. So I recommend that if you have a sound problem, you give a system reset a try. It won't fix everyone but since it did fix mine, I know it will help others who had the same issue as me. In my situation, it is possible that one of the apps I installed didn't play well with JB 4.2.2 and caused sound problems or maybe even it was just Jelly Bean itself. But resetting the system and uninstalling all my apps did fix this aggravating problem that has been going on too long. I will reinstall apps only one at a time and wait several days to make sure it wasn't one of them causing the problem.
Also note that on my system, sound for games was normal and good. It was only the sound for media (music, video) that was low.