The sounds of your footfalls on the treadmill belt can indicate if you have an asymmetrical running pattern and are landing harder on one foot. While none of us are perfectly even, a marked difference can lead to injuries from your low back to your feet. Pay attention to the sounds of each foot striking the belt, and if you hear (and feel) one side coming down louder, then chances are high you're working one leg significantly more than the other. Altering your running gait is difficult and can lead to other injuries, so instead of tweaking your feet, I suggest your play with your pelvis. Focus on your core moving evenly through space as you run. If you imagine your pelvis being pushed from behind to propel you forward your legs will follow and hopefully they will follow more evenly.
Aside from listening to the rhythm of your feet, check the volume of your foot strikes — the lighter the better. If you're pounding into the belt, the solution to your problem is once again your core. Support your torso, lifting through your low abs, and see if you can run with stealth. Work these five core exercises into your strength-training regimen; they are great for runners and will help you decrease both the volume and impact of your foot strike.
So experiment with running sans music next time you're on the 'mill and let us know how it goes.