I was installing PC based CCTV systems for a living back in mid 2000s, I had a restaurant customer have their mind blown by pandoras music service while demoing the PC. They asked "can I hear this over the restaurant speakers?". Yeah, I got 3.5mm to RCA to BNC in the car which I can run to your AMP.
Now he was really happy, felt like he got launched into the 21st century.
"Can I pause the music and make an announcement to my customers?". I made him a shortcut on the desktop to sndvol32.exe aka sound mixer so he can talk over the speakers and then resume the music and he was again floored.
"I want my staff to be able to do this but not here in my office but at the front counter".
Easy. "Give me a paper clip and 30 minutes". Went to the car to grab some stranded Cat 5 and an old beige two button IBM mouse. Ran the cables from the office to the reception area, soldered a cat-5 pair to the left mouse clicker, drove a sheetrock screw threw the mouse body to mount it on the wall like a taxidermied deer head. Back in the office I soldered the leads to the paper clip so I could have a solid connection to the LPT port on the PC (I didn't carry DB-25 back then).
It was easy from there: googled "VB6 Volume Mixer". Download that, open the checkbox for mute stereo mix, copy one liner into a timer, set interval for 100ms, check if the pins on the LPT are shorted, if so mute stereo mix, unmute the mic.
This was written in the field without any pre planning. I still have this customer to this day. What a great tool for us "jacks of all trades".