It helps a little bit in walls that don't have doors.

I wish we had insulated our interior bathroom walls.

Pooh Bear

We insulated just about everywhere to help with sound dampening (I'm a bit of a nut about this though;). We are also using 5/8 drywall which should help too.

My builder mentioned a layer of cork between drywall and studs. He said that works much better than fiberglass in the air space. I think the sound deadening drywall probably is better than both.

It is one of those items that "fell below the cost line", but I have wished I had gone ahead and installed interior insullation.

Idle thought - besides the sound dampening, does the insulation provide a few extra minutes of time in case of fire? If you use cork board, I wonder if that helps or hurts in case of fire. I don't know, but something to at least think about ...

We insulated between walls and floors. Should help make the house very quiet... and eery if no one else is there.

thanks all for the follow up.... seems like insulating between walls would be money well spent.

In our house the bedroom and kitchen are back to back.
We did not insulate between the two walls but wish we would have for soundproofing. The refrigerator sits on the same wall as the head of our bed and I can hear it every time it kicks on or off.

Insulating the wall cavity with fg makes virtually no difference in sound transmission if you don't do anything about "flanking" sounds. I have found the most improvement in reducing sound transmission in real homes (as opposed to rating charts) to have come from using solid core doors cut tight. On sensitive areas, I've used resilient channel and an extra layer or two of drywall or damping drywall systems such as Quiet Rock.

Here is a link that might be useful: STC ratings

Interior insulation is usually low density and thin so it has little effect on direct sound transmission through a wall but it does reduce the sound that enters through receptacle and light boxes, unsealed cracks, etc. because the sound energy must bounce back and forth through the insulation in the stud cavity before it exits through another crack on the other side.

Of course, careful location of electrical and duct outlets can be as important as acoustic insulation and solid core doors with sound seals (especially at the floor) also help. Sound can also bypass walls through attic/truss spaces. As you raise the acoustic performance of any room element you must raise the performance of the other elements.

Fire resistance is not an issue since in a home fire spreads rapidly though open stairways and hollow core doors without automatic closers rather than through walls.

Insulation might make a very very small difference in interior walls but not much. If you are interested in actually spending money on reducing sound transmission - I would say look at using solid core doors (cut close like worthy recommended). If you really want to spend more money after doing that - you may want to consider an extra layer of drywall on one side of the wall (hallway side) or better yet - use QR or GG between drywall sheets.

The extra mass of the drywall is a much bigger improvement than fiberglass between the studs. (does not address flanking noises though which fiberglass doesn't either).

We used a double thickness of 5/8" drywall on the wall separating our exercise room from the master bedroom and it's virtually impossible to hear the treadmill, TV, elliptical or fans in the master. But regarding worthy and anthem's suggestion for close-cut doors, be aware that that can cause circulation problems for forced air HVAC systems.

We used fg in most interior walls for temperature control because we have individual thermostats for radiant heat in almost every room and we typically keep the unused rooms set at 50 degrees. We have little problem with sound transmission but that's probably due more to our solid core doors as mentioned above.