Problems with playing through DAC/headphones

Using a MacBook Air myself for iTunes and YouTube listening. I listen either
through the main system (MBA USB>MF V-Link 192>Schiit DAC>Amp) or via
headphones using Audioquest Dragonfly in the MBA USB port. I find there are 3
locations to check: 1) Apple Logo (upper left toolbar)>System
Preferences>Sound; 2) Audio Midi Setup: after clicking on icon lower right of
dock, click on Window in upper tool bar, then Show Audio Window; 3) I use
BitPerfect. You might be using some other program. There too I need to check
Preferences to make sure it's recognizing either the V-Link or the Dragonfly.
Sometimes I can switch between modes of listening with no problem. Other
times i have to manually select the proper "source" in one or more of
these locations. Maybe when changing sources (iTunes vs YouTube), something
resets inappropriately. Obviously, make sure volume control in various windows
is not set to mute. Hope this helps.

Thanks so much for the replies! I will give these a try tonight and report back.

Checked all my settings, and everything seems set to play through TEAC. I wonder if, as you say, something resets or doesn't switch over appropriately, when I toggle between iTunes and Chrome or Firefox.

Don't toggle. Quit the program.

Funny thing is, when I was using a Nuforce ICON HD DAC with the same headphones, this problem never happened. I could listen to all sources on my computer with no problems without having to exit the programs. This must be unique to the TEAC.