...also, a solid wood door will NOT reduce sound as well as a composite door. You can order a door that is "skinned" with real wood. The key is mass. Lots of mass and the composite doors will be significantly heavier than a solid wood door. Also solid wood can resonate terribly (and transmit the sound to outside of the theater room). There is a reason that speaker cabinets are NOT made with solid wood. Just keep that in mind.
I went to the local lumber yard / supply store and had the manager lookup the heaviest exterior, gasketed door they could order. I also ordered it iin flat panel oak with jam extensions to fit my thick wall and with ball bearing hinges to support the weight. It was quite a chore getting it down into the basement! I think I paid about $275.