[SteveRB]

Hey.

Just looking for recommendations for surface treatments to apply to a long wall between mine and my neighbors suite. I can't remove the drywall but I can put something on the drywall and paint...

Biggest issue for me right now is volume. I want to be a good neighbor  but I also want to listen at decent volume. There have been no serious complaints but I'd like to be pro active.

[ACHiPo]

Unfortunately sound attenuation is not easy, requires mass, and is not cheap.  While it may not be practical in your situation, adding another layer of sheetrock, overlapping seams, etc. could get you ~25 dB of attenuation if the wall is airtight, and there are no holes, e.g. Electrical outlets that allow sound to leak. 

Your best bet is to make nice, see when is an allowable time to listen loud, and stick to it.  If that doesn't work look at headphones.

[Hipper]

You should consult 'Sound Proofing' companies. This UK firm gives some ideas:

http://www.customaudiodesigns.co.uk/soundproofing-acoustic-insulation-products.htm

I would think having speakers on the far wall opposite your neighbour's wall would help.

In addition I think bass trapping helps. Afterall the biggest issue is usually bass and using bass traps absorbs bass energy. It will also make your room sound better anyway.

[JLM]

Easy solution as mentioned would be to turn down the volume, and play nice (invite neighbor in for a listen).

Next easy solution would be to turn down the bass.  Bass has much more energy with frequencies that more closely match the resonant frequencies of building/construction elements.  I run two different systems in the same room and the speakers with less bass (F3 of 70 Hz versus 30 Hz) don't transmit nearly as effectively through the house (quite noticeable).

The physical concept is to reduce sound transmission and therefore increase (improve) the STC (sound transmission coefficient) of the wall assembly (the total of all elements that make up the wall) in the walls adjoining the neighbors would normally be recommended.  An effective solution would be to explore the cavity (around electrical convenience receptacles (wall plugs) of those adjoining walls to determine if they are insulated, if not insulated see if the landlord would be willing to have them filled with cellulose to increase the STC of the wall assembly.  This would also help transmission through the electrical convenience receptacles.  A more serious and effective solution would be to look at using "Green Glue" (high STC construction adhesive) to add a layer of drywall to the wall(s) adjoining the neighbors and insulate/cover/remove the electrical convenience receptacles.  In new construction or renovation work the better solution would be to build an insulated room within a room to structurally isolate and thereby short-circuit sound transmission.

Adding mass does help reduce sound transmission, by decreasing resonant frequency and increasing the energy needed to excite the greater mass of the wall assembly, but typically isolation is a more practical solution.