12 ga on 15a is not normal but its also not dangerous at all. The breaker is what protects the wire from seeing to much power. A 15a breaker could never overpower a 12ga wire. You could technically run some 10ga to a 10a breaker if you really wanted to. Whether or not an inspector would throw a fit is another story.You should have clearly and explicitly stated something about wire size then.
Just because you think you've never seen anything smaller as far as mains electrical wire gauge as you're running audio cable, doesn't mean anything. 12ga wire is not used on known 15A circuits normally at all. It is absolutely not the norm, so assuming it is and providing no warning is quite dangerous.
Also, it's Romex
Also, the majority of homes in existence are more than 20 years old......