The movie came out in 1979 in germany. Somehow, many previously silent parts of the film have Carpenter music added in the german mono dub. Rumors is that Carpenter may have done it, thought publishers deny it as there is no proof on the web he did it.
It's all done professionaly and mixed to perfection, so my guess is that Carpenter didn't like the silent part, and added music to make the film drag less.
It's the same cues as the music soundtrack, so I'm guessing it should be easy to reconform the US mono soundtrack by adding them and mixing them in. I can provide you the german mono if you want :)