Sound playing while muted ....?

I have a LG TV connected via HDMI as my primary monitor. I have my sound set to “Internal Speakers” on my Mac Mini. And the sound is muted.

So one would think there would be no sound. But certain things (email sending in MailMate, messages coming in in Messages) play notification noises via the LG TV.

Now…I could disable the notification noises manually within the programs, but I think it’s bizarre that sound is playing via a device that’s (in theory) not active.

Anybody ever had a problem like this? Is there a solution?