MJR, before you get any further we need to make it clear that there are two basic goals for a sound room, and that the two are ALMOST completely independent of each other (except that accomplishing the FIRST goal will normally cause you to work harder in order to accomplish the SECOND goal)
First is sound ISOLATION, commonly (incorrectly) called sound PROOFing - this means keeping sound IN or OUT of your space. The result of doing this is the same in both directions; if you keep sound IN, you keep it OUT as well. There is no portable, cheap, or easy way to accomplish serious isolation. There are, however, less expensive ways; you start by learning enough of the basic physics of sound control NOT to waste time, money or materials.
Second is sound CONDITIONING, or acoustic treatment. This means making the room SOUND GOOD, and is done AFTER you decide on, and accomplish, the level of ISOLATION you actually NEED - acoustic treatment of a space does very little to change the isolation, but isolation construction WILL change the acoustics in the room - this is why the isolation needs to be taken care of FIRST; otherwise, acoustic treatment that worked before will CHANGE, especially on the low frequency end.
That said, are you concerned at all with keeping sound IN/OUT, or just with getting a GOOD sound WITHIN the room, or is it BOTH?
It's not possible to solve a problem until you know WHAT problem you're solving; all the above is intended to help you decide WHAT your problem(s) is/are, so we can help you find a solution... Steve