I found a solution!!! ;D
The system has got a lot more complicated.
I did render a seperated audio file where the music is already reversed and I use the world location to determine from where the reversed track should be played when turning around. (and same story when turning around to move forward again)
Thx everyone for thinking and reading! 
And if some having the same problem and reading this and need some more explanation about this system, just ask here! 