Distortion Issue

Hello, on a song I've recently been recording, I'm having an issue where I will export to .mp3, play it on my computer, home stereo and car stereo. The song will sound fine on my computer (played at full volume) and my home stereo (haven't tested at full volume because, well, neighbors). However, when I play it back at full volume in my car, there is significant distortion on the vocals as if it is clipping. Now, I would be willing to accept that my car speakers aren't that great and move on but it doesn't happen on other music (even other stuff that I have recorded).

I record using Audition (I guess that's a given) on a Macbook Pro via a M-Audio Fastrack Pro USB interface. The session itself is a stereo session with a bit depth of 32 (float) and 44.1kHz Sample Rate. The song has two vocal tracks (melody and harmony), both recorded using a Shure SM58 mic. Together (panned right 50 and left 50) they peak at about -3dB. I also double-tracked an acoustic guitar strumming chords (also recorded with the SM58). The tracks are panned 100 right and left and each track has a Parametric EQ. I then route both to a bus track which has a Tube Modeled Compressor on it. The guitar peaks at about -12dB. The overall mix peaks at around -3dB.

The session is then mixed down to a .wav and saved at 44.1kHz and 32 bit. I then create two files from this master: a .wav (16 bit) and a .mp3 (192 Kbps CBR) both at 44.1kHz and normalized to -0.01dB (I do also perform some compression prior to normalization). I use the .mp3 as my test on the different speaker systems.

The distortion appears to only be on the vocals (perhaps because that is the loudest part) and only when played back at max volume. Initially, I thought it was a bit depth issue in the conversion of the 32 bit .wav down to a .mp3 (dithering?). I've also considered maybe there's over-compression (although it would seem to me an over-compressed track would just sound warbled). The most frustrating part is that I've used this same process on other tracks with 3 guitars, vocals, and drums that sound fine. Any help would be greatly appreciated.