Dynamat is for sound deadening and the foil can act as a radiant barrier against heat. It doesn't take much Dynamat to deaden a panel. Beyond that, there are better and lighter materials for sound insulation which it sounds like you are after.
Exactly. Dynamat, KilMat, or the cheap tar-based stuff from Lowe's is used to stop vibration and it does so by adding density to the panel. Heavy jute padding is best for sound deadening, and the foil covered bubble wrap type of heat barrier seems to work well, especially when combined with a thin layer of foam. You can add ceramic beads to truck bed liner, latex paint, Lizard Skin, or rubberized undercoating as an alternative option.
Each of these products achieves multiple things to varying degrees, but only truly excels at one.