Both host and guest are Parabola x64 distro, but host is openrc and guest is systemd. After my guest has been running for about a day, I stop getting audio output. Speaker-test hangs and there's no sound from any application. Audio is restored if I reboot.

All channels shown on alsamixer are at max volume. I haven't altered or created any system audio settings other than increasing/decreasing the alsamixer indicators.

systemctl restart alsa didn't seem to do anything.

alsa-state.service is running normally whenever I check its status. I used to have alsa-restore.service running instead, but I had the same problem with it. Restarting these services didn't fix the problem. Of course I did not use them at the same time.

I am not part of the audio group but audio has always started off working despite that.

I'm wondering whether there is some kind of pulseaudio interference. Pulseaudio isn't running but I cannot remove libpulse because it is a dependency of many applications I need (browsers, wm, etc.) On the other hand, libpulse is the only pulseaudio component I have installed.

Which virtual software suite are you using to run the guest?

Your host uses your computer's audio device, the guest typically uses virtual hardware supplied by the virtual software.
Two different audio devices, your virtual software suite may be designed to suspend some features after a set period of time or whatever. Hard to diagnose without knowing which audio device it uses.

KVM and virt-manager. I'm not 100% sure I understand the question.