Is the idea to treat the room or prevent the sound from disturbing others? Simply treating the room will not help in this regard. Also, covering an entire room with fabrics and foams will make it a very dead space for music. You may want this in a studio, but not in a playback room. Playback rooms for music should be of a more "live" type. Treatments are good to flatten out the room response and may help a little with cutting down the sound outside the room, but it is not the main purpose. Again, if you can (doubt it seeing it is an apartment) decouple the listening space and make the walls more dense.
sc
I like a fairly dead playback room.
I listen to alot of material with mucho spacial information in it from two-mic live recording.
Putting sound absorbing materiel on the walls, floor and ceiling will do just that. It will help, not stop, sound reflection and transmission.
The problem with sound abosorbing materials is that they aren't effective for low frequency sounds, espeially sub-sonics.
These get re-radiated as walls, studs, furniture, and such as they resonate from them. (even in adjoining spaces)
scervin is dead on, you must decouple bass to keep neighbors or 'rents happy.
Speaker spikes or a plinth with spikes and spacing from the walls is a very first step.
Dissimilar materials of varying densities is another good way to decouple.
Sandwich and glue high-density foam, styrofoam, plywood, etc. together to make sturdy decouplers