In the Skype Meetings App, how can you control your USB/external headset volume?
This one has both stumped and annoyed me for a while.  I had been unable to find a solution to this online but, eventually, I figured it out so I figured I'd share my discovery.

To modify the volume you need to click on the speaker icon on the Mac menu.  



After clicking on the speaker icon, a little menu will show up showing the available audio devices.  Select your headset and then the hot-key volume controls and the volume slider will adjust the volume on your headset.

Once you've modified your volume settings, you can set your primary audio device back to what it previously was.  Your settings on your headset will remain the same, even if you adjust the volume on your primary speakers.

It's pretty simple once you figure out but wasn't immediately obvious.