Ethel was about much more than volume. (Sample the new Decca CD of her 1950/51 singles. The novelty tunes wear a bit thin one after another but Ethel sings the GENTLEMEN PREFER BLONDES numbers far better that Channing ever did.) Ethel had near flawless diction, and perfect pitch. If her style of performance seems weird now remember she was primarily a comedienne and got her start in 1930 when that style WAS the style.
Barbra is (or was) at her best in her early 60s Columbia albums (and the FUNNY GIRL Ocr on Capitol.) Her 70s/80s pop recitals are less exciting but there's no denying that the lady can sell records!
As for Patti, her diction isn't as good as Ethel's and her voice lacks Barbra's warmth but at her best (EVITA, SWEENEY TODD) she is a powerful stage presence.
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