Currently all of our soundtrack tunes are looped so that they work seamlessly ingame. Switching to another track mid-race would sound really jarring. Each music track is 3-4 min long. How many laps are you doing?
As for the jukebox:
-Yeah, the soundtrack cannot currently play along with your custom music. This is just how the software works. We would have to code some more functionality for it, which is kinda low on the priority list. It will be done at some point though
-The music player certainly plays the custom songs at a lower volume, so you may need to turn it up for them. But I haven't noticed a reduction in quality