Heat and sound require different insulations, if you want to reduce both you will need 2 layers. Outfits like
Cotronics and
Pacor sell ceramic papers and paint which will reduce the heat. Lead or asphalt sheets will reduce vibrations and are available from audio suppliers,
B-quiet and
Parts Express are a couple.
You also might try
Cool and Quiet, they make insulation for the hot rodders. Just make sure the insulation is rated for the heat and petroleum products it will come in contact with, a generator compartment is pretty nasty.
John