Is it normal to lower the car stereo volume?

I think you're a prole.

The music must distract you from doing your parking thing. Quite normal.

It means you're concentrating, which is not a bad thing when it comes to driving ;)

I do the same thing!

Yes I would do the same. Music distruct me from the road so I lower the volume to think better.

If this is a repeat, sorry! It's because you have to pay a lot more attention. Driving tends to be an "overlearned" behavior, unless you're a new driver, because you do so much of of it. Therefore having the radio on louder while driving usually isn't a problem. (If road conditions get bad, say, really foggy, you probably turn the radio down, though.) Parallel parking usually isn't done enough to be overlearned, so you naturally turn the volume down because the parking maneuvers demand so much more of your attention. That's all an oversimplication of something from my Human Performance class, but I hope gets the general idea across.

I get really distracted by music. I would do the same thing (if I listened to music while driving, that is).

Lol I thinks it's just a natrol reflex. Stop worrying so much lol