Your best bet is to go to Sound Cloud and search there, when you find something you like, go to it's page, copy the URL and put it into the Roll-20 search and 99% of the time you will get the sound you wanted. There are literally thousands of clips and tracks on Sound Cloud and you can layer them in Roll-20 to produce a soundscape that rivals any movie. I ran a scene in the spring where the party sees a distant campfire across a frozen plain and hears russian folk music, the snorts and murmerings of reindeer and a light wind. A thunderclap breaks the bucolic scene and suddenly there are drums, two different evil chants and the sound of a large crowd. Then a bump in the lighting and a drop from the GM layer and they see hundreds (I would scale it down in the future) of orcs descending on the hapless Ruskies.
