Ron,
Any decent grade of 1/2" hardwood Ply should work, I really don't think marine is necessary since it shouldn't be getting wet.
As for sound proofing this site, defender, west marine etc. all carry foam made for the purpose just search for sound proofing. Alas, the stuff is not cheap! You'll also want a roll of heat resistant tape for seams.
The trick is attaching it. The stuff I had, had an adhesive backing but I wanted something a bit more secure so I used plastic tube inserts stuck into holes punched in the foam (with a pencil) and screws with washers to secure the foam. The insert keeps the screw from binding up in the foam and act as a depth stop for the screw.
Brian