I booted my Archlinux PC today and opened Mumble, and found that I couldn't hear anyone, so I opened some music and I couldn't hear that. After turning my volume to the maximum possible level in pavucontrol, I could just about hear the song okay. However, there's still a problem here, in that my gamecom 780 (USB headset,) is normally much louder than this, and was working fine before I rebooted.
To show how severe the issue is: normally, I keep my headset at about 18% volume, and grooveshark turned down to near the minimum volume, and that was far louder than the current max volume on both with this issue.
I found out, that the soundcard in my PC (as opposed to the one in my headset,) with cheap earphones works somewhat better. I don't know if it's working as normal though, because I don't ever use it, and that does seem a little quiet as well (could just be normal for these earphones though.)
I did update all my packages before shutting down yesterday, and I think that could be the cause, but I couldn't say which packages updated, as there were many.. It's fairly safe to assume that if the updates are the problem, Pulseaudio or ALSA must have updated..
I went into alsamixer and tried to turn the headset up there, except I found that there was only one volume slider for my headset's output, and that was the mic loopback (I think.)
I tried following the advice given on the Archilnux wiki ALSA page, which said to use a tool to reset the codecs. My headset's built-in soundcard wasn't listed there though, and doing this didn't change anything. There was another option listed on that page, that said to use an alsa plugin to boost the sound, however I don't want to use that as it causes distortion and clearly there's some other issue causing the sound to be quiet and a workaround like this wouldn't solve that.
I also tried deleting my PA config, to little effect.
I'm on my Windows partition right now, and the sound works perfectly here, which means it's definitely a software problem, at least. Will reboot into Linux (don't know why I started this post in Windows...) and edit this post with additional info that is requested.
Long after writing this, I reinstalled Arch Linux; surprisingly, the problem did not go away, and this seems to indicate my problem is a regression in one of the packages responsible for producing sound (likely pulseaudio...)