A couple weeks ago my speakers stopped working. When I reboot, they work perfectly. But eventually some mysterious, and I suspect nefarious, piece of software takes control. (On a related note, Youtube videos seem to stop working when the speakers die). The drivers are up to date, and I ran a Malware Bytes scan and nothing came up.
I first found this question and answer: https://superuser.com/a/496589/79629
In the properties of your audio device, there should be a checkbox option labelled "Allow applications to take exclusive control of this device". IIRC it is enabled by default; disabling it and rebooting should resolve your issue.
At first I thought that worked, but then the speakers stopped working again, even with that option disabled! These two screenshots were taken immediately after each other.
My next thought was to identify what was actually using the speakers. I found this second question and answer: https://superuser.com/a/253751/79629
The Volume Control application, accessible by right-clicking on the volume icon in the notification area next to your clock on the taskbar, or by running sndvol, should identify what applications are presently using your sound card.
However, it only shows me the system sounds.
Side note, Windows 8 has a hilarious tool called "Find and fix audio problems" which literally just checks if you're an idiot and have the volume muted.