When you are in the huey in the passenger seat, and the song is played, the faster the heli is going, the slower the song gets, abit like the the "theory of relativity" emphisised a bit too much.
If it is coming from the helicopter, then passengers wouldn't experience the Dopler effect IRL. People on the ground should notice it though, and people in vehicles traveling in the opposite direction (if I remember Dr Richards high school physics class correctly).
Its not coming from our understanding of physics, its simply a bug .
It has already been reported several times, so please make sure you search the forums before posting a bug.