I have a bunch of reclaimed wood of varying thickness, most are 1.5-2" thick. If I cut them to fit between my studs, would that serve as a good sound insulator under the finished wall? Would I have to seal each seam? This is between a bedroom and bathroom.
Using wood for sound proofing sounds like a ton of work compared to the gain you would get from it. The easiest thing to do (and likely more effective) would be to put in fiberglass batt insulation to fill the dead air space and then double up the sheetrock on both sides (or whichever side is open). Adding mass is the key, preferably at a widely different resonance level from the framing. Sheetrock fits both bills very well. If you want to do a simple experiment, put your ear on a stud length 2x4 and have somebody tap lightly on the other end and you'll see what I'm talking about. If you really want to cut down sound transfer, use a wider floor plate and stagger-stud the wall as well.
Agreeing with Comintern. So long as you're up to code, greater stud distance and even screw spacing (yes, that's right) will give you a higher STC (sound transmission coefficient) by decreasing connection points for sound to travel through the wall. The screw spacing is more important for the second layer since it physically connects the studs to the outside of the wall. 2 layers of 5/8" gypsum with staggered seams and the greatest screw spacing will do this. Green glue, quietrock, silentfx, and brazilian channels would all be things to check out. This may be an old thread but it was next to mine so I thought I'd shed a little light just in case :) Happy DIYing!