Problems with playing through DAC/headphones

Just bought a TEAC UD-501 DSD DAC and have been listening to lossless files via iTunes and DSD via the TEAC player. I am listening primarily through headphones, and the sound is great. For some reason, though, if I try to load a video via youtube or if I try to listen to music clip on an internet website, I hear nothing via the headphones. I have a MacBook Air and check the sound settings -- the TEAC is selected as the primary output, so I can't understand why I can hear music via iTunes and other dedicated players but not via my web browser. Any insight into this problem would be greatly appreciated! Thanks!
Is the TEAC player running in the background? Maybe it's set to exclusive or direct modes that disables other programs from using it.
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.
With DoggieH. on that one. Do you have sound if you try to play a YouTube vid first and only? (no iTunes played previously during that session??). I do notice even trying to eject the external hard drive where I store my iTunes library that if I haven't QUIT both iTunes and BitPerfect the drive won't eject because one or both are still using it...this despite thinking I have stopped and exited iTunes and BitPerfect. Definitely worth trying to quit iTunes before switching to YouTube. Good luck.