In the Skype Meetings App, how can you control your USB/external headset volume? While connected to the Skype Meetings App, you are given the option to specify specific devices for audio output and input.  However, since this is a selection within the Skype application, normal volume control doesn't seem to function.  Instead, increasing/decreasing volume seems to change the volume settings of the primary audio device.
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.