fill it with soft materials, rugs, wall art, plants, couches. the more you have in it the better as far as noise reduction goes. although I can hear my phone ring when I'm in my bathroom like it is right next to me. pretty interesting sometimes.

also if you have large windows like I do you can cover them with a thick cloth to help the sound. although I have nothing on mine

Do you mean within a single unit or controlling acoustics between units?

its a single unit.. about 4800 sq ft. and were trying to make it in to 3 'bedroom' areas...

Some sort of white noise usually helps, like a fan blowing.

It isn't easy living in a loft, you might want to look into some good quality ear plugs.