I had this exact problem and have fixed it, but had to try a multitude of things. Ultimately, I think re-installing your graphics driver may be the solution.
If that doesn't work, you can use a program like This. It allows you to create a hotkey that locks your mouse onto your main monitor
I didn't add it just because that apparently broke when they removed VOIP so they definitely know about this one and will be fixing it as soon as they add VOIP back in. For now I suggest downloading a really useful program called volume2 - https://github.com/irzyxa/Volume2 - you can bind anything on your keyboard or mousewheel to mute/+/- volume of any individual application