Alsamixer settings haven't changed, and I don't think any audio packages have been upgraded in the last few days.
Check pacman's log file, then you can be sure.
I even checked to make sure the speakers were still plugged into the line-out jack (they are). I'm wondering if that POP could have damaged the audio port on the motherboard. Is there any way I can check, or otherwise diagnose what the problem might be?
Have you tried booting from a live cd (ubuntu or something else that's meant to work out of the box)?
If you can't convince your system to produce any sound at all, I'd guess your hardware is broken (that loud POP sound might have been a capacitor that simply exploded). If that's the case, you should ask an expert who can actually take a look at your hardware.
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