There's an unused theme, but what makes you think it was supposed to be played in Terra Tubes ? Just because Terra Tubes reuses the surf music doesn't mean the unused theme was necessarly going to be Terra Tubes' theme.
Also the Megadrive version re-use the Intruder Excluder music for Terra Tubes, so two levels in row have the same music, which is not particularly great - I think the NES version is supperior.
Personally I think that as the unused theme loops rather shortly it was probably intended for some kind of race.
Before youtube, hearing the full Speed Bike track was beyond most people's skill.
Even if you hear it, you're so concentrated on the game that it's unlikely you'll really listen to it. Also NSFs appeared long before youtube.