Sorry, I just wasn't sure if anyone saw my reply so I was checking just in case.
I think that I am learning a lot thanks to you and others so far, so I am feeling closer to the confidence level I need to begin the project.
So to summarize and make sure that I understand correctly (sorry for the length!), please tell me if you agree with the following conclusions:
1) Walls - Although certain materials improve STC ratings when inserted into a cavity of a wall, some of these materials actually have a high NRC rating when exposed (i.e. when not behind a solid reflective material). Regular 2.5" fiberglass insulation batts for example don't have an STC rating but have an NRC rating of 0.9-1.0, making it more effective than the typical foam-based sound absorbing material, not to mention a much a cheaper alternative. In my situation, as the fiberglass batts (or cotton-based ones you mentioned) will be located between the concrete wall and the heavy curtains (which are absorptive rather than reflective), the combined NRC values of the curtains and fiberglass batts should suffice for my sound absorption purposes. This coupled with the sound blocking properties of the brick/concrete walls should probably cover my total soundproofing needs.
2) Doors – Two solid core doors on the same door frame with wether stripping around the inner part of the frame to seal them properly is the best way to go rather than dealing with hollow core doors. The faces of the solid core doors are reflective, therefore two things needs to be done: i) the surface of the door facing the inside of the room needs to be covered with sound absorbing material to avoid sound waves reflecting back into the room and; ii) if the two doors are less than 2” apart (or even 2.5” as in my case) a resonance chamber will be created, and therefore sound absorbing material will be needed to fill in this space.
Did I get it right?
By the way, regarding the process I discuss in point 1 above, do I still need to worry about an excessively “dead sounding” room or would this still be O.K.?