Spock's Brain Opening Cue

[Methuselah Flint]

I gather the opening cue for Spock's Brain has interference making some of it unusable, hence we got some of The Enterprise Incident music.

Am I right in thinking, however, that the cue is used during the decontamination procedure in Let That Be Your Last Battlefield?

If it was unusable for the former, why did they use it for the latter?

Also, was the interference to do with the recording/mic?

Thanks.