volume lowered when playing music

When I'm in my (old) car, I play music using one of those cassette thingies that plug into the audio output on the iPhone. So, the iPhone (6 plus running most recent iOS 8.4 -- although this is not a new problem with 8.4) thinks I'm using earphones. Often, when I first plug in the iPhone the volume delivered to the output is at a much lower volume than the volume control indicates.

I've had the same thing happen when using the same iPhone in my daughter's much newer car where the connection is made using the lightning connector.

It's as though the music has been quietened because of some other microphone/speaker activity and then not made louder again.

Indeed, I can fix the problem by making a phone call and then hanging up after the call connects. My daughter speculates that simply chatting with Siri will fix it, too, although I've not tried that.

But I don't think such steps should be necessary. The music should be at the advertised volume not some oddly lowered amount.

Is this something unique to me, or are others seeing it too? Does anyone have a more direct fix?

Thanks,

Dave