I went through the arduous process of flashing a stock rom back onto the phone and tested the audio; there was no difference. I paired my old M7 to the car and played the same songs in comparison, and there was no question that it sounded much better. I think it bears repeating that the M7 is running 4.3.x, would that make a difference?
The issue comes up with upper treble frequencies, where it begins to sound very compressed and distorted, much like white noise. To eliminate all doubt, I played some CD's with the same songs, and they sounded crisp and fine.
Any additional help with this would be greatly appreciated. Streaming bluetooth audio in the car is very important to me and the inability to do so at a level of quality I would consider acceptable makes me rather inclined to return the phone. Is this a potential fix upcoming in 4.4.3? Thanks in advance.