I'm curious of what would happen if you attach an external USB soundcard and set it as the output device for Teamviewer. I'm sure the system volume would work like a charm while being able to play the remote session's audio through the external soundcard (at what volume, that I don't know). So, again, I'd say there's some bug here with the mixing of Teamviewer and system audio (Teamviewer's volume messing with the system's "master volume" level).