I am insulating the walls in my duplex for sound. I need advice on what will be most effective.
Do the materials used become a health hazard once installed? I am skeptical of foam due to gases.
Before you start insulating, you need to know what your goal is. To know your goal, you need to know the ability of your current wall to resist the transfer of sound.
The ability of a wall to resist the transfer of sound is measured by its Sound Transmission Class (STC).
In the first web page, notice on the second row that adding insulation in a typical wood stud wall will raise your stc from 35 to 38. Look at the next row down. If you add one layer of drywall to each side you increase the STC to 41.
In general, mass increases the STC faster than insulation. But it may be difficulty to add dry wall to your neighbor's side of the wall. There is a relatively new product out that I have specified several times called QuietRock. A single layer of Quietrock on a typical wood stud wall can give you an STC of 56!
So the direct answer to your question is: insulation may not be the most effective way to block sound in your wall.