Rock Wool is often used in apartment buildings partial for its ability to dampen noise.

Stick to your criteria, and then nobody can pull the wool over your eyes.

@Jeff Cotton the only thing I would add to what @Bill Gulley said is that you get what you allow.

We use steel wool to fill the holes, then cover the exterior as normal.

Fan is wired with a humidistat and timer, so we need not worry about tenants either not tuning it on, or turning it off when they leave.What you do not see in that picture is behind the walls - when renovating a bathroom we put 3" rock wool (Roxul Safe-n-sound) batts in all the walls to provide sound deadening and, in some instances will use 5/8 drywall and resilient channel.  

Eventually, I stuffed some steel wool into the crack and I'll go back there in a week or so with a bag of concrete and a tub to make a full-on tenant-satisfaction pantomime out of filling that crack and protecting her against the terrifying concrete-tunneling squirrels.

I have replaced some and it looked like it was stuffed with rusted steel wool.

Once your Buyers cotton on to your deals not being cheaper than they might get from any Agent, you're out!

Alex Cotton I agree with your methods.

The sound dampening I did was very effective and included the following:- rock wool insulation- acoustical putty pads on the backside of the electrical boxes- mass loaded vinyl- 2X sheets of 5/8” drywall with green glue in between the drywall- acoustical caulk on all remaining seams