I actually experienced this also, like you said not all the time, but enough to be annoying. And I would look through the volume settings and everything appeared normal. No explanation.
Anyhow, I regularly have a speaker plugged in through the headphone jack. So I would just reach behind, and unplug it for a moment, and then plug it back in, and it would be resolved. -Yes, another tedious work around, but much quicker than having to do a full Restart.
The computer knows when something is plugged in or not and changes the output in settings. My guess, is that the action of unplugging and replugging is enough to get the system thinking about it, and reset the sound card driver.
So maybe if you don't normally have anything plugged in and just rely on the internal speakers, you could test this by having a cheap set of old school ear buds laying around. Next time it happens, plug them in and take them out. See if that's a quicker work around. Good luck.