Muting PC audio when doorbell rings
I'm afraid that it's more of a hardware question, but I'm offering you a DIY answer :)

How about light instead of sound? 

Install some simple fixture on the wall (or desk) where you are sitting (beside the PC?) most often, put the red bulb (or whatever color you like and can make you notice easily). The wiring shouldn't be hard to guess, but if you're not really into this, you could ask another question (or modify current question) to make it work. Simple and easy. And it will not force you to make any changes regarding USB ports and signal capture.  

The weakness of this solution is that the light needs to be noticeable - in case you go to a toilet or something, you may lose that.