Your MTH track already has a roadbed built-in. Adding roadbed underneath will make it even higher than the surrounding scenery and structures. And finding roadbed wide enough to support the existing roadbed may be difficult. Most track systems with built-in roadbed are fairly noisy. The hollow roadbed surrounded by rigid plastic tends to amplify some noises. Some have stuffed the hollow area with foam rubber or similar, but that is a laborious process. Probably the quietest 3 rail track is the Atlas track with solid rail on plastic ties.
Of the roadbed materials, cork is the most common. It is cheap, and works fairly well when it doesn't dry out (think twice about using it in the desert SW). The foam roadbeds sold by Woodland Scenics and Vinylbed also seem to work well. The Vinylbed has a more rubbery composition. The foam with an adhesive built-in is not as easy to use, and doesn't necessarily hold when you want it to, and holds when you don't want it to. Note that any track/roadbed system gets noiser when the ballast is glued down, especially when white glue is used.
Hope this helps