You're welcome! Here's the Google-fu (in case anyone wants to try it for other requests in the forum): I entered the phrase, "'I'm a Big Fat Mama' movie" in Google ("I'm a Big Fat Mama" is enclosed in quotation marks so that the search engine will look for the exact phrase, and will show pages that refer to movies).
The fifth link in the results points to a RT page about the "Yum Yum Girls."
I entered the title in quotation marks with the year in Google, selected "Videos," then "Tools," then set duration to "Long" to look for any full-length videos of the movie online. Only the one with the running commentary was available from YT, but that was good enough.
In YT, I moved the mouse cursor over the time bar of the video and scanned the smaller picture to see if there are any scenes featuring Edna Thayer auditioning, and there it was.
Bonus: check out the 1972 NYT article that comes after the RT link in the results:
https://www.nytimes.com/1972/07/19/archives/a-65-yearold-trouper-makes-it-to-broadway-in-the-automat.html
