Hear ambient sound while listening to music with headphones on OSX I don't want people to yell at me when they want to say something while I'm using headphones on my desktop computer.

I'm thinking an app could just listen to the ambient sound and lower the volume when it is loud. Apparently, Awareness does something like this on iOS. What about OSX?
There is an app in the macOS app store called "Dumbo" that lets you monitor room audio from your mac mic when you have headphones on. It doesn't automatically lower the volume, but you can adjust the mic input level as well as the iTunes level to get the mix you want.

Here's the link to Dumbo in the US Mac App store.