yes I have the same issue and it drives me insane. I'm not sure what causes it, I thought it was whatsapp because there's a weird audio API call from within the music app - when you resume a pause song it opens whatsapp but i'm not 100% sure.
I really hope this gets fixed and issues like this makes me wonder whether anyone at HTC/Microsoft tested Windows Phone 8 before it was released :/
Update: My phone has just been doing this, set to "ring and vibrate" yet no sounds as if it's silent. I just checked settings > ringtones and sounds and 'ringer' was set to off, even though the phone was set to ring and vibrate. I turned this on and sounds are working again! Looks like a bug!