My built-in audio output doesn't work

Hi,

I just noticed my built-in audio output doesn't work anymore. I've tried all sorts of headphones, but none of them seemed to work. Right now I'm using an external USB soundcard, but I'd really like to figure out what happened here. I haven't installed, upgraded or updated anything at all: it just happened. Do you guys have any suggestions about this? What kind of data should I look for to help debug?