I have heard this reported before (it's not the movie soundtrack, it supposedly happens on the original Broadway cast album with Ethel Merman) If anyone can pinpoint where, I would like to hear it.
On the WEST SIDE STORY cast album there is an audible click just after the Scherzo on the "Somewhere Ballet" - just before the vocalist starts to sing "There's a place for us..."
These flubs were relatively minor and would often be swallowed by the RIAA roll-off and the noise of the vinyl (or shellac for the old 78 RPM records.) Now when re-mastered for CD, these become more apparent...but that's part of the charm of these albums. Like a theatrical performance they are "live" and anything can happen.
Also, in the early days of recording the performers were back a bit from the mic so they could project without taxing the systems. Now they are so closely mic'ed there is no space between their mouth and my ear. I prefer the open more spacious sound of the earlier recordings. They have a you-are-there-in-the-theatre feel to them. But with the open mic policy, there is more chance of outside noises creeping in.
Cast albums are NOT "soundtracks."
Live theatre does not use a "soundtrack." If it did, it wouldn't be live theatre!
I host a weekly one-hour radio program featuring cast album selections as well as songs by cabaret, jazz and theatre artists. The program, FRONT ROW CENTRE is heard Sundays 9 to 10 am and also Saturdays from 8 to 9 am (eastern times) on www.proudfm.com