[Nick Zwar]

It might have been that Howarth helped out Carpenter on the synth music uncredited, as they often collaborated back then. Carpenter has said:
” I cut his [Morricone’s] music into the film and realized that there were places, mostly scenes of tension, in which his music would not work … I secretly ran off and recorded in a couple of days a few pieces to use. My pieces were very simple electronic pieces – it was almost tones. It was not really music at all but just background sounds, something today you might even consider as sound effects.”

And those brief “background sounds” are the only non-Morricone pieces you have in the movie. (But they were not on the MCA if Varèse Sarabande or Quartet releases of the soundtrack, but on the re-recording Alan Howarth did.)