Thunderstorms in Mountains
I presume you mean if one were a substantial distance away from a lightning strike in the mountains, would it be heard better or worse by another observer the same distance away in the plains and foothills? It depends whereabouts in the mountains this lightning strike occurs and where the observers or listeners are. 

If it occurred between two rocky ridges forming a V shape with the open end directed toward the plain, it is likely that the observer on the plain would hear it more clearly than an observer or listener the same distance away in the mountains. The very mountain walls which reflected the sound toward the plain would act as barriers to listeners placed, let's say, 10 miles away in the mountains to either side. This is called acoustics.

One can also imagine an acoustic layout in which the mountain observer would hear the strike more clearly and loudly, but on balance the plains observer would be more often in a position to hear the thunder clearly. My own observations of thunder in the mountains and coastal areas of Brunei and Sarawak would seem to bear that out