Lumber is covered in mold/mildew all the time from being wet, etc. during storage. I know we just put it up without a second thought (this was before the current hoopla regarding the mold plague). I suppose that a water/bleach mix, 4 to 1 will pretty much take care of it (wear a good respirator goggles and old clothes, rubber gloves, etc. I personally don't think it is an issue on the lumber as I think it will only be dangerous if you have the conditions present in your house to promote its growth, leaks, too much moisture, etc.
As for soundproofing I have used resillient channel with good effect. You mount this across the studs on both sides with isolating padding in between. Also make sure to seal ALL drywall joints with acoustic calking (not regular calking) before joint compound. This means ceiling joint, adjoining walls joint, floor joint (needs tight fit, not an inch gap on bottom as is common), and calk all joints where sheets meet each other before drywall tape/mesh. See: http://www.soundproofing.org/infopages/channel.htm