In the 1950s or 60s, a filmed musical featured the song::So They Say
(that falling in love is wonderful, etc.)".  Who sang the song in the
film?

Hi Rowdy,

The song is called "They Say It's Wonderful".

It was sung by Betty Hutton and Howard Keel in the 1950 film
version of "Annie Get Your Gun". It was also a best-selling record by
Perry Como. The words and music are by Irving Berlin.

Lyrics:
http://ntl.matrix.com.br/pfilho/html/lyrics/t/they_say_its_wonderful.txt

Annie Get Your Gun, soundtrack - Internet Movie Database
http://us.imdb.com/Soundtracks?0042200


Search strategy:

musical "So They Say"
://www.google.com/search?q=musical+%22So+They+Say+%22

Best wishes,
robertskelton-ga

The answer was acceptable in all respects but one: The title of the
song is "So they say" not "They say it's wonderful" as you expert
says, regardless of his references.

Unfortunately, as researchers, all of our answers that are not based
on personal experience must rely on authorative references, or a
consensus of opinion. I looked further into the proper title for this
song, and this is what I discovered:

Based on a consensus via the web, the title "They Say It's Wonderful"
appears to be the more correct answer.

A Google search for:

"They Say It's Wonderful" "Howard Keel"
://www.google.com/search?q=%22They+Say+It%27s+Wonderful%22+%22Howard+Keel%22

returns 176 websites, whereas only three are returned from this
search:

"So They Say" "Howard Keel"
://www.google.com/search?q=%22So+They+Say%22+%22Howard+Keel%22

each quoting the lyrics to the song, with the title being "They Say
It's Wonderful".

Checking the index of a book via Amazon, "The Complete Lyrics of
Irving Berlin", which is surely the authority on the matter, "They Say
It's Wonderful" is the only title of the two mentioned:

http://www.amazon.com/exec/obidos/tg/detail/-/0679419438/ref=lib_rd_zi_34/002-0491035-7304038?v=glance&s=books&vi=reader&img=34&jumbo=1#reader-link

For what it's worth, I played the show once, and the title in the
books for the orchestra is "They Say It's Wonderful", as well.