There are a few options for sound insulation. I personally installed the vettenuts or exotic vettes insulation package which are precut to fit each area. This kit also cuts down on heat as well.
Others have installed Noico material. Others install b-it mats. It depends on your preference. Not one will significantly reduce the noise level by more than 10 decibels, but it will improve the cabin noise to be sure.
I found that adding extra insulation in the void in front of the rear wheels will reduce tire road noise and reduce cabin noise.
I've added a rear compartment top and partition which helped a bit more in noise reduction and keep the rear speaker directed to the front.
We use our C6 for 90% of our road trips, so cabin noise reduction was a must.
If you want to see all the options, Google images by typing C6 Corvette noise insulation and you will see the options available and what to look for.
Good Luck