If you want to deaden sound you need a door with density and a good seal and threshold.

Inside doors really aren't that expensive, about $90-$100. You'll need to cut the door to fit your opening but you can use the old door as a template. You'll also need to chisel out the hinge locations but you can use the old door to see where these go as well. Finally, you'll need to drill out the door handle and again you can use the old door to determine where this should be.