Barcarolle by F.Schubert for Flute and piano duet, 10 pages (7 pages flute and piano score, 3 pages flute solo) 2:25 min, with downloadable mp3 just for audio help.

"Joy to the World" is a popular Christmas carol with lyrics by Isaac Watts and music by G.F.Handel. As of the late 20th century, Joy to the World was the most published Christmas hymn in North America.
For Flute and guitar duet.
Easy.
With downloadable mp3's for audio help (one with both the Flute and guitar and one with guitar only for practicing the flute).
With guitar tablature. With melody line. With lyrics.
Pages: three pages (score and Flute part)

Mauro Giuseppe Sergio Pantaleo Giuliani (27 July 1781 8 May 1829) was an Italian guitarist, cellist, singer, and composer. He was a leading guitar virtuoso of the early 19th century.
This is the second of the set of 16 easy and pleasant duets for flute and guitar composed by Mauro Giulani at 1810.
For Flute and guitar duet.
With downloadable mp3's for audio help (with both the Flute and guitar and one with guitar only for practicing the Flute).  With guitar tablature.
Pages: Six pages (2 pages both Flute and guitar, 1 pages Flute part, 1 page guitar part, 2 pages guitar with guitar tablature part), 3' min.

How Great Thou Art is a Christian hymn based on a Swedish traditional melody and a poem written by Carl Boberg (18591940)  Sweden, in 1885. How Great Thou Art was ranked second (after Amazing Grace) on a list of the favourite hymns of all time in a survey by Christianity Today magazine in 2001.

For Flute and Guitar duet (Guitar in double D tuning (D,A,D,G,B,D)).

With standard notation and tablature.
Easy to early intermediate.

Pages: SIx (6) pages. (3 pages Melody/Flute and Guitar with tab, 1 page Melody/Flute part, 2 pages Guitar and tab part).

Flute Trio by Leonard de Call  (19 March 1767 19 February 1815).
Three movements: Allegro, Andante and Allegro.
Pages: 49 pages, (Score 19 pages, Flute 1 part - 10 pages, Flute 2 part - 10 pages, Flute 3 part - 10 pages).
With downloadable mp3 for audio help or practice.