occamrzr06 is on the right track. If you know how to get to that console, assuming you don’t have 3rd-party software installed for sounds (Creative, etc.), you can check everything there.
Sometimes my ‘device’ deselects and goes to my USB headphones, killing my speakers. Sometimes YouTube mutes for no reason. Sometimes the “level” (found by double-clicking the ‘device’) is set to minimum.
Who knows what the heck causes it, but if you have Windows tones, and your sound card/speakers aren’t dead, you can at least check everything else from that console.
However, if you have 3rd party software installed, you must find that icon in your toolbar and open that application to ensure nothing got hosed there. I’ve had 3rd party drivers get hosed by Windows updates on a number of devices, including sound cards. It could also be a browser add-on problem (usually correctible by a reboot if it’s not corrupt).
But usually most of the problems above kill ALL the sounds.
I ran the trouble shooter and it said all devices are fine. I know its something simple- I hope ;)