Very low sound volume (XPS 13 9343)

I have an XPS 13 9343 that I've had for five months (came prebundled with win8.1, but I installed Windows 10). When it was brand new the sound volume (both speakers and headphones) was very loud.

For some reason, about two months ago, the sound volume mysteriously decreased significantly. I tried the usual things (rollback drivers, update drivers etc), no change. But the volume was still ok, just not as loud as it was in the beginning.

Today, after a hang and reboot, the sound volume is now very low, both on speakers and headphones. I've read all kinds of tips, and tried everything but a system restore (something I won't do, since I won't spend a day reconfiguring and reinstalling stuff at work).

There are no driver updates available, and rolling back drivers does nothing. I've checked the mixer, and everything is at 100%. This is getting really annoying and seems like such a stupid thing, something that down the line is going to make me never buy a Dell again.

Has anybody had the same problem, and did anyone find a fix or even a reason for this?