AutoGoose

This tiny desktop app will automatically lower the volume of Spotify (or other music players) when other applications are playing sound or you are using your microphone. Your music will automatically fade-in again after your call or other application has stopped playing sound. Ideal for online meetings, watching a quick video while listening to music or playing games. This process is also known as real time audio ducking or auto ducking.

Regarding anti-virus warnings: This app needs to inspect your audio levels on an OS level, it is harmless, but some antivirus scanners seem to mark it as a false positive. Please follow the instructions of your antivirus to exclude the app.

You can keep it on your screen for easy access in mini-mode by collapsing the settings: