You need heat resistance and sound dampening both, and they are pretty much seperate insulations. I am working on this in several areas of my mh right now so here are a few links:
Cool and Quiet
J. C. Whitney
I got spray heat insulation and sound deadener from Parts Express
for use on the cowl, doghouse, and floor. I haven't applied these yet so I can't comment on the effectiveness, but liked the spray for all the nooks and crannies in this area.
Most of the performance sites will also offer insulation. Just make sure the adhesive is high temp.