Use LATEX caulk. Go to Home Depot or Lowes and buy a tube of their Latex painter's caulk. Its about $2 a tube. The tube will also say "ALEX" on the side.
$0.02 I don't disagree, but the sound isn't an issue for many of us. Half of my foam layout has plaster atop the foam; it's very quiet. The other half has no plaster, and it is indeed less quiet, but no, it doesn't matter much. I glue my track onto painted foam roadbed using latex caulk or similar water-based products. I use inexpensive latex house paint for my basic earth color. Here's a before view showing painted foam roadbed. Just for good measure here's an after view of the same area. For what it's worth, I did a short Ground Cover Tutorial in my RailImages for foam terrain. Maybe it will provide some useful ideas. I discovered that it is a good idea to remove the thin (1 mil?) polyethylene skin from the foam before gluing. Glue doesn't adhere to the film very well at all. I agree. Latex caulk is also useful for filling gaps and small holes in your foam terrain. Once shaped, painted and covered with soil and ground foam, the caulk will be invisible. Here's an example (before): ... and after. For my layout, I prefer to use common and inexpensive materials. I think they've worked reasonably well so far.
