I am going to use UltraTouch cotton insulation inside of a standard wood framed wall, drywall on the inside, wood siding on the outside. for other reasons the wall is almost 8" thick. I'm confident 5" is plenty thick enough for insulation purposes, but I want to get good sound damping also, so does it make sense to keep adding insulation beyond the 5" for soundproofing or am I wasting my money? Thanks.

Up to the point where you are packing insulation in and increasing its density, "more is better" is basically the rule. 8" of insulation in an 8" wall cavity will do a better job than 5" - over time, probably better enough to justify whatever the cost of the extra insulation may be.

However, if your only purpose in insulation is for soundproofing, then the extra insulation will do little for you. Walls transfer sound through their framing as well as through the dead air space between the studs. Unless your framing is staggered and built as two separate walls, adding insulation for sound control does not do very much.

This is very helpful. If I have staggered walls is the more insulation the better rule then apply regarding sound proofing, or are the staggered studs the most important factor, and the gain from say 5" to 8" of insulation pretty minor from a soundproofing point of view. Thanks.

For soundproofing, staggered stud walls or two separate walls will do more for you than a single stud wall packed with insulation. The easiest and most common way to build a sound-resistant wall is to use 2x6 for top and bottom plates, then set up your studs on 16" centers as normal. The big difference is that you will have twice as many studs as for a single thickness wall as the studs for one side will be staggered between the studs for the other.

Thanks a lot for all the tips!