Thank you for contacting Plugable support! Sorry to hear about this issue. I’d be more than happy to assist you.
This is pretty common with Windows. Setting a default will only keep it that way until Windows detects a change, then all bets are off if Windows is going to remember the default, or change it without asking. I recommend disconnecting the dock from all devices and power, then reconnect everything - power first, then the rest.
If there’s no change in your options then I recommend rebooting the computer. Then you can see our blog post here https://plugable.com/2016/01/15/set-default-sound-device-in-windows/ which might be of some assistance, though it sounds like you’re doing exactly what I would recommend, but that the option isn’t there.
If the reset of the dock and the reboot of the system still are having the issue, next we may want to get some diagnostics and see what happens. We have a troubleshooting tool that helps us find out what is going on in situations like this. Please plug the dock into your computer, then go to this page and follow the instructions there: http://plugable.com/support/plugdebug
Please don’t hesitate to let us know of other questions.
Thanks again for contacting Plugable support and best wishes!
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