Since version 1.2.1 ufo2map is generating a footsteps file. This file includes all the textures which faces were marked with the surfaceflag footstep. These textures can be merged with contrib/scripts/footsteper.sh into base/ufos/terrain.ufo.
These soundfiles are played when an actor walks over a texture with a terrain entry. This file is global, so all maps are affected by it. So set the footstep flag wisely.