Uncommenting
load-module module-alsa-sink
load-module module-alsa-source device=hw:1,0
in `/etc/pulse/default.pa` did the trick.
Edit: However, after uncommenting the lines mentioned above, I ran into all sorts of problems, for example vlc started crashing. I now removed the complete folder `/etc/pulse/` and now everything seems to work fine, including sound and mute button.