I'm going to look into the legit sound bug, thanks for examining it so thoroughly!
Regarding the not-sure-bugs: The sound in general has been a low priority, I wanted to first focus on the gameplay, networking and graphics. That said, I would like to improve it in the future and I would love to get some happy music for the game some day, that is the long-term plan. The down-pitch of the engine is meant to break up the repetitiveness a bit and can be explained as the player shifting gear (at least sometimes :)