Interior doors don’t typically have weather stripping, but adding some around the perimeter of the doorway can help muffle sounds.
For example, using peel and stick foam rubber. It can actually form a very tight seal. It is also very affordable and easy to install.
The function of door weather stripping is surprisingly effective at blocking out noise. In fact, install a door sweep or under-door draft stopper to the bottom of the door to seal off the gap along the floor.