We're building a modular house this spring and an option is to have insulation between the interior walls. Why would one do this? I can image if you had separate heating zone per room this would be useful, but we're getting forced air most likely, so I'd guess it isn't as useful. Is there a significant sound dampening? If so, I'd consider.

Thanks

There is some sound deadening...I don"t know if I would call it significant. If you have teenagers who play music you don't like, at sufficient volume to be heard on the Moon, all the insulation in the world won"t help you, but sounds like normal conversation will be at least muffled and possibly completely absorbed.

If the wallboard on one side of the wall, preferably the noisy side, is attached using resilient furring clips, in addition to the insulation, the sound-deadening performance of the wall would be perhaps significantly improved.

I do not know if I would use insulation between interior walls unless you really need the additional quite. Perhaps in a library room, or multi-media room that is near bedrooms. Like Richard said, the noise will still be heard, but a little less on the higher pitched sounds.
Reason I do not think its a good idea, With all the advancements of multi-media these days it would be darn near impossible to run new cables or wires through walls that are insulated.
Just my opinion.

if you got spray foam it would deff. reduce any niose by 75 percent

KR could you tell me where you got that that information? Never heard of 75% before. Perhaps I missed something.

Some people in OUR area have begun to insulate the interior walls for one reason: So the heat can be turned down in one particular room and held in there. Some of our clients do not leave their entire home open and therefore do not heat the entire home. Thus : The overhead or floor ducks only heat the one area and it stayed warmer in that area even at a lowere setting.