I started seeing this issue after I started using a 7.1 headset, never happened with stereo. I thought it might be caused by some soundmods I had installed, namely JSRS and Enhanced Soundscape, but they have no effect. I also noticed severe audio artifacts and frame drops in towns, not quite as unplayable but definitely noticeable, probably something to do with the buildings and reverb. First person in a vehicle, especially going at a higher speed, consistently brings this problem. Switch to third person and instantly fixed. Does not occur in 32-bit.
System:
Windows 7 Home Premium 64-bit
FX-8320 CPU
16GB 1866 Memory
HD 7970 GPU
Patriot Viper V370 7.1 Headset