I tried all the suggestions here but the slide sounds still only play in complete silence. My impacts are fine though. Did you get those working Andy?
Anyway, while fiddling around I discovered that something had gone wrong when I parented the emitter to the kickable.

Even though I had confirmed the Blender "make parent?" dialog it never happened. I know what caused that failure now. Not sure if this is a Blender bug but if parent and child to be are not in the same layer Blender won't do the parenting. Something to keep in mind...