Tom,
In a +10 year old special issue of MR Magazine, How To Build Realistic Reliable Track, one chapter addresses quiet roadbed and the author provides a chart with sound evaluations of eighteen roadbed configurations. Cork over camper tape over a sub-roadbed of plywood ranked the quietest.
Some modelers use diluted matte medium as a ballast adhesive as it does not cure as hard as white glue.
Thanks,
Old Goat