Plaster Walls

I have plaster walls and there's no need to put up sheet rock. Use carpeting or oriental rugs, hang drapes, and tapestries, and use soft furniture and bookcases to dampen the room. You'll need to use acoustic panels on the walls to treat the room for sound absorption and/or diffusion. Sometimes a panel is needed on the ceiling.

Look at the GIK Acoustic website for samples and advice. If the room is for listening only, you can use DIY foam panels; not as attractive but inexpensive.

Plaster/lath is much denser, more rigid and thus: less resonant, than the Sheetrock over stud construction, of later homes. The former retains less energy/returns less to the room, than the latter, which I've personally found; makes those rooms easier to treat. The shape and dimensions of any given room will have more bearing on the Sabine times, absorption or diffraction treatments necessary, for good audio reproduction. Plaster/lath also better reduces the sound & noise that is transmitted to/from adjacent rooms, or the great outdoors.

Yeah plaster should be fine. Plain sheetrock is the worst. I had to move back to the basement with stone walls because the modern sheetrock was thrumming. It you do ever decide to sheetrock, put it over the plaster.