Foot drag sound not very audible when using keyboard nav

[jbphet]

When using keyboard navigation to interact with the simulation, if the user moves the foot one increment forward or backward, the foot drag sound is barely audible.

I demonstrated this to @emily-phet and @Ashton-Morris at the 4/23/2019 sound design meeting, and we decided to create a GitHub issue but leave it unassigned and low priority for now. It's generally a bit hard to hear subtle sounds when used in conjunction with the screen reader anyway, so there may come a time when we take a look at this, but for now it's not a priority to address.