First of all, some info about your hardware. Type on Terminal:
QUOTE: "Can anyone guess why this randomly stopped working, how to get it working again, and how to stop this happening in the future?"
Why? Because of PulseAudio.
If I understood you correctly, you have PulseAudio installed.
The removal of PulseAudio might be an ideal solution for your problems.
PulseAudio is a sound server and network server, that is why reconfiguring firewall may produce a trouble.
1. Firewall (iptables) are already in Linux kernel. You can install a wisard for configuring it (this is what you did, perhaps).
2. Both PulseAudio and ClamAV (and any other antivirus for Windows malware) are security risk.
It is still possible to remove PulseAudio from Mint 7,
but it might be impossible to remove PulseAudio from Mint 8.
In short, to fix the problem, you may try to remove PulseAudioviewtopic.php?f=48&t=32725
!!! Fresh clean installation !!!
Install another Mint 7 in dual boot and try my howto to see how it works.
See also my howto for dual/multiple boot here:viewtopic.php?f=48&t=33657
If you cannot manage to make your sound working with Mint, you may try antiX, or else.
antiX (LiveCD, or USB, or install to HD)http://antix.mepis.com/index.php/Main_Page
It is a rolling releasehttp://en.wikipedia.org/wiki/Rolling_release
Install (with Synaptic) ESound, Adobe Flash with plugs, Skype, and reboot.
Everything works "out of the box" with default settings.
antiX has ALSA inside, not OSS4, and, nevertheless, sound seems to be much better than with Ubuntu or Minthttp://antix.freeforums.org/post12648.html