A few film adaptations that actually improve upon the stage originals:
CALL ME MADAM - The script has been streamlined to eliminate a needless subplot about hydro power. Some of the funniest scenes were added to the screenplay (Merman's scene presenting her credentials is a comic gem.) The movie also distributes the songs better so that it is not all Merman. The additions are all improvements over the songs they replaced.
FLOWER DRUM SONG - film is too long and has some dull patches but story-wise holds together better than the Broadway original.
FUNNY GIRL - despite the loss of "The Music that makes Me Dance" and "Who Are You Now?" the film script plays without the clumsiness of the original stage production.
GREASE - the movie version is far more enjoyable than any stage production. Most of the new songs are welcome additions, and even if you lose a lot of the true 1950's flavour, GREASE was never intended as documentary.
1776 - not so much better than the stage show (which is nearly perfect) but the film preserves so many original cast performances and the movie cast album (on LP but not CD) has Howard Da Silva who was not on the Broadway cast recording due to illness.
SILK STOCKINGS - a fairly flat stage musical was transformed into a delightful film.
SOUND OF MUSIC - the Austrian location footage really makes it a beautiful film to watch but also most of Ernest lehman's changes to the song placement and dialogue lead-ins was masterful. (The recent Broadway revival adopted these changes and went one better moving "Lonely Goatherd" to teh concert scene. Well why not? I does sound like an Austrian folks song, and it gives ACT II a lift.)
VICTOR VICTORIA - The movie came first and is much funnier and better performed than the awful Broadway version. I suppose te movie is not truly a musical - it's more a comedy with songs/production numbers used in the nightclubs but none of the songs advance the plot.
Good film adapations include SHOW BOAT (1936 version), OKLAHOMA!, KING AND I, MUSIC MAN, MY FAIR LADY, PAJAMA GAME, DAMN YANKEES, HOW TO SUCCEED..., BELLS ARE RINGING, WEST SIDE STORY, FIDDLER ON THE ROOF, EVITA, CHICAGO.
CABARET is a great film but has little in common with the stage musical.
At the other end of the spectrum, the "what were they thinking???" films....
BYE BYE BIRDIE, PAL JOEY, CAMELOT, HELLO DOLLY!, MAN OF LA MANCHA, PAINT YOUR WAGON, MAME, ANNIE, BEST LITTLE WHOREHOUSE IN TEXAS, A FUNNY THING HAPPENED ...FORUM, A LITTLE NIGHT MUSIC, A CHORUS LINE, THE WIZ, ...it seemed as if Hollywood was deliberately trying to kill off the audience for film versions of Broadway musicals.
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