Problem: 

No sound

Problem details:

When I attempted to test it, I got a message dialog box saying:

"The device is being used by another application. Please close any devices that are playing audio to this device and then try again."

Attempts:

Opened "speaker properties" window
(right-click speaker icon -> select "playback devices" -> select device -> select properties button)
Selected 'advanced' tab, and unchecked the boxes under exclusive mode. Did not help.

Solution:

Uninstalled and re-installed sound drivers:

1. Opened "device manager": (Start button -> type "device manage" (without quotes))
2. Uninstall sound drivers:  under "sound, video, and game controllers" - right click, select uninstall. Did not check-off/select "delete the driver software for this driver".
3. Re-installed: While still in the device manager window, select 'action' (upper left corner of window), and then select 'scan for hardware changes'. If there is no problems with your hardware, windows should 'find' your speakers and begin installing device drivers automatically (takes a few minutes).   (To help prod it along, you can try troubleshooting hardware/speakers, but it should go automatically.)

Cause/prevention: 

Unknown. Sounds like malware took over the speakers or something like that, but so far, MBAM (Malwarebytes) hasn't found anything... If anyone has any ideas, let me know.

Tags:
No sound, troubleshooting sound, this device is being used by another application sound