My machine developed an interesting 'issue' a little while ago. For some reason, it decided I didn't want to hear anything I tried playing in Windows Media Player - other apps would play OK. I am not sure what caused it, but I did find a way to get the sound back in WMP today.
Yesterday I was looking at the Audio control panel - and I noticed that if I went into the (so-called) advanced settings, that it was showing the mute-all button was selected. Now, that sounds like a simple user error, right? But it wasn't.