Apple - Teamviewer is lowering my Macbook Pro's audio

This sounds like the audio ducking feature, commonly used by video chatting applications to temporarily lower the volume of other sounds.

To turn it off, go to Applications/Utilities/VoiceOver Utility.app > Sound

Uncheck "Enable audio ducking".

TeamViewer only ducks the audio device you selected. So if you don't want this feature you should be able to simply select one that you do not need (in the Teamviewer settings, not in macOS).

E.g. I use a RØDE NT-USB external microphone as my sound card. I selected Built-In Line Out/In as my audio device. Now TeamViewer has no idea what to do. :)

It's an ugly hack but victor's solution didn't work for me.