Best practice for Windows setups is to set Windows to use the on-board sound card as the "default Playback Device", which leaves the interface(M-Box) to be controlled by Pro Tools exclusively. The simple/cheap solution to playing other sounds(WMP, Youtube, etc) thru your monitors is to add a small(cheap) mixer to your setup. All you need is 2 stereo line inputs(1 set for the M-Box outputs and the other set for the soundcard output). This is a suitable option on a budget:
https://www.amazon.com/Behringer-502...9812165&sr=8-2
Another option is to add a monitor/source selector like the Presonus Monitor Station 2, Audient Nero or similar. Yes that's a lot more money, but having used a Presonus central Station for years, I can't live without it(or similar). A real plus is that some of these allow you to connect the computer via a digital SPDIF connection(which sounds 100 times better than the mini TRS analog output)
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