When you get into a car, on some of them you hear the sound that plays, and it turns the music volume down, but I don't think the music volume goes back up. Does anybody else here notice this happen? If it's a bug, can I report it anywhere?

Hello, pure accident that i found your post since it's the first time i am on the gog page of Cyberpunk 2077.
I reported this bug directly to the gog support, and they told me to report it in the Cyberpunk 2077 CD Project Red Forum in the 'TECHNICAL' section. I did this on 14 April of 2021 (i can't post links to nether of these, you can use the search function in the cyberpunk 2077 forum and search for

[PC - Audio Bug] Unterschiedliche Lautstärke des Radios im Auto

but it's in german.

The support told me he would forward it to the developers. I played the game three times after release and not more since then, so the bug seems still to exists.

I could go around this bug by exiting the car and enter it again. By the way i didn't realize the connection to the entrance chime, maybe because i exclusively used one car (Mizutani Shion "Coyote") in all three playthroughs (but i can't remember if it has a entrance chime).

PLEASE do me a favor and report this bug, it annoyed me and i want a working radio when Phantom Liberty is out :)

Doesn't this issue occur when the radio transition from music to a news broadcast? Have you tried changing radio station or turned the radio off and on again to get the volume back?

I have noticed this bug, but it's a non-issue for me, as I either keep the radio off, or use a custom radio station.
Such fun flying around in a monster truck while listening to Darude - Sandstorm. <3