Hey, Pit.
So today I decided to watch a Youtube video, however as the video commenced I realised that no sound was coming out of my speakers.
Confused by this, I opened up iTunes and tried listening to some songs, to make sure that it wasn't a problem with Youtube itself. No sound came out of my speakers.

Later, I opened up a Guitar Pro file, and found that sound works with that.

So, I can hear MIDI files, but I can't hear anything else like MP3s etc.

Anyone know what the problem is?

Edit: I could hear everything perfectly the day before, and I did not touch/change anything that I know of to make this occur.
My Last.fm
USA Fender Stratocaster | Roland Cube 60 | VOX ToneLab LE
Open the mixer (usually comes up when you click the volume icon) and see if anything is turned all the way down that shouldn't be...that's usually what it is when I can hear regular music but not midi files, so maybe the reverse happened to you...