One thing I noticed is that your own footsteps are pretty loud because military boots aren't necessarily made for sneaking around. Wouldn't it be nice to have an alternative pair of shoes for moving around inside citys? I for one can walk almost completely silent on paved roads or inside the house when I'm wearing sneakers.
So my idea would be that you can find a pair of sneakers just like you can find a ghillie, and putting them on affects your movement and the level of noise you make.
In general you could only run on paved streets and inside buildings (sneakers aren't particularly made for the wilderness :)). Audibility would be very low while walking, and considerably decreased while running.
Make shoes take up 3-4 inventory slots so it doesn't become a no brainer keeping 2 pairs. And maybe give them a small percentage to break your ankle while running or some other form of penalty.