How do I seal my bedroom door for soundproofing/reduction?
Cut a piece of wood as a filler strip to fill the gap at the bottom of the door. Screw it to the bottom of the door, and when you remove it nobody will see the screw holes since they're on the bottom of the door. 

Next add adhesive backed smoke gasket to to top and sides of the door jamb. 

Finally, if you still need additional sound proofing, you can put egg crate foam on the inside of the door. I'm not sure how to temporarily attach it though.