Reddit reviews Green Glue Company 10730 Noiseproofing Compound, 12 Pack

I'm not OP, but it's another layer of soundproofing. You apply it between the layers of drywall.

http://www.amazon.com/Case-Green-Glue-Noiseproofing-Compound/dp/B000SKWD8Y

One of the cheapest ways to significantly cut down on sound leakage is to double up the sheetrock on the walls with a layer of Green Glue in between them. Make sure to caulk all corners, joints, and anything that could possibly let air leak through it. If you can, get a door that seals shut when you close it, but if you're on a budget, just try to get a heavy, solid door, and put weather stripping around it, sealing off all the cracks.

As far as filling the walls with insulation, I know Roxul makes some great sound baffling insulations like Safe N' Sound, or other similar ones that will also help you cut down on sound leakage significantly. And if you have the time and effort, I've heard it helps to put staggered cross braces between studs to break up the large number of identical resonant chambers in the walls to several smaller, different sized resonant chambers. Granted, I've never done it before, but adding some cross braces between studs doesn't seem like a whole lot of extra work if there's a potential for it to make the room sound better. Maybe someone can chime in on how effective it really is.

Floating the floor will help a lot for isolating the bass, but you can also build some smaller platforms for things like bass and guitar amps and drum kits. Or you can get something like the Auralex Gramma pads for them if you don't want to build them yourself. They won't isolate as much as an isolated floor, but they'll help.

Then finally work on acoustic treatment to make the room sound "nice" for rehearsal and recording. Make some corner bass traps out of thick rockwool panels or Owens Corning 705/703 (depending on what you can get your hand on), hang some scattered absorption panels on the wall, leave some scattered wall surfaces bare, and maybe add some diffusers to the room to add some natural reflections and break up the big, empty, rectangular space. But acoustic treatment for a live room depends on a lot of things, so that's something you'll probably have to figure out on your own.

Luckily the 15 degree roof pitch will help a lot in taming the room and creating a more acoustically even space, which is convenient. Often studios will design all of their walls and flat surfaces to be at different angles to reduce the prominence of room modes that can create some odd sounding spots in the room. The angled roof won't make it perfect, but it will make it better than a roof that's parallel to the floor.

I'm no sound expert or anything, but the expanding foam is much less dense than whats in the solid core doors, so I'd imagine less effective.

Upon further research, it seems like expanding foam isnt well regarded as a noise dampening solution in general. There is a product called Green Glue (which I could only find in 12 packs for $200...), that some people seem to revere as a gift from the soundproofing gods. Price aside, I'm not really sure how you would apply it inside a door, since it looks pretty thick (would be hard to distribute evenly).

Personally, I'd go with the simple option of a new door. But, you have some options now, and its up to you.

Do you own the condo?

In additoon to upgrading the doors and windows, you could tear down the drywall and make sure the insulation is high quality. Then install new drywall by hanging it on whisper clips which separate the studs from the drywall. Then you apply anti vibration green glue and a second layer of drywall.

For sound dampening, I've read that Green Glue is the best.

Easiest and most cost-effective way is to use this stuff:

http://www.amazon.com/Case-Green-Glue-Noiseproofing-Compound/dp/B000SKWD8Y

If your attic doesn't have existing drywall, you'll need to hang two sheets of drywall with GreenGlue applied (as per instructions) between. Do the same with the floor and whatever door is up there. Don't forget to seal.

If you've already got drywall you'll need to strip the walls to the drywall, clean it, apply the glue, and hang another sheet (then of course refinish the walls and seal).

I would advise against the whole process though. It's going to be absurdly hot/cold in an attic. There will be sound bleed. It'll be cramped. I just don't think you're going to be happy with the results after going through the cost and effort to professionally soundproof your space.