“Sheet music” has nothing to do with bed sheets, but there are jokes.
 
”Why did the composer spend all his time in bed? He wrote sheet music” was printed in the Home News Tribune (New Brunswick, NJ) on October 14, 1998.
 
“What do songwriters compose in bed? Sheet music. Then someone comes along and does a COVER” was posted on Twitter by Happy | Healthy (and others) on January 27, 2016. Authorship is uncertain.
   
“Farting in bed: sheet music” was posted on Twitter by dr. puma pants on January 16, 2015. “Farting in bed is just sheet music” was posted on Twitter by Th3Jay (and others) on July 31, 2020. “What do you call a fart in bed? Sheet music! #LaughoftheDay” was posted on Twitter by Jimmy RevJim Olsen on November 7, 2020.
   
               
Wikipedia: Sheet music
Sheet music is a handwritten or printed form of musical notation that uses musical symbols to indicate the pitches, rhythms, or chords of a song or instrumental musical piece. Like its analogs – printed books or pamphlets in English, Arabic, or other languages – the medium of sheet music typically is paper (or, in earlier centuries, papyrus or parchment), although the access to musical notation since the 1980s has included the presentation of musical notation on computer screens and the development of scorewriter computer programs that can notate a song or piece electronically, and, in some cases, “play back” the notated music using a synthesizer or virtual instruments.
 
Newspapers.com
14 October 1998, Home News Tribune (New Brunswick, NJ), Kid Stuff, pg. C8, col. 4:
Why did the composer spend all his time in bed?
He wrote sheet music.
 
Twitter
darline_mumford
@darline_mumford
Why did the composer spend all his time in bed? He wrote sheet music.
12:33 PM · Sep 18, 2009·Twitter Web Client
 
Twitter
NothingButJokes
@NothingButJokes
Why did the composer spend all his time in
bed?
He wrote sheet music.
4:00 PM · Aug 1, 2011·NothingButEntertainment
 
Twitter
Greg Witt
@DrumAvatar
Did you hear about the musician who spent his whole life in bed? He wrote sheet music.
3:40 PM · Sep 6, 2011·UberSocial for BlackBerry
     
Twitter
dr. puma pants
@athleisure_monk
Farting in bed: sheet music.
7:43 PM · Jan 16, 2015·Twitter Web Client
 
Twitter
Happy | Healthy
@HealthyZindabad
What do songwriters compose in bed?
Sheet music.
Then someone comes along and does a COVER.
11:58 AM · Jan 27, 2016·IFTTT
   
Twitter
Andrea Andrews
@AndreaAndrews21
@barlowlookalike joke for you…What do songwriters compose in bed?
Sheet Music! Then #SomebodyElse comes along and does a #Cover.
12:11 PM · Jan 27, 2016·Twitter for iPhone
 
Twitter
PUNS
@ThePunnyWorld
What do songwriters compose in bed?
Sheet music.
Then someone comes along and does a COVER.
9:18 PM · Nov 13, 2016·Buffer
 
Twitter
PUNS
@ThePunnyWorld
What do songwriters compose in bed?
Sheet music.
Then someone comes along and does a COVER.
8:18 PM · Feb 23, 2017·Buffer
 
Twitter
Zac Galifianakis
@ZacGalifianakis
What do songwriters compose in bed?
Sheet music.
Then someone comes along and does a COVER.
5:30 PM · Mar 7, 2017·Twuffer
           
Twitter
Uncle George
@FixitUncle
Why did the composer spend all his time in bed? He wrote sheet music.
7:06 AM · Mar 21, 2018·feed_140
     
Facebook
Dad Jokes
April 9, 2018 ·
What do songwriters compose in bed?
Sheet music.
Then someone comes along and does a COVER.
 
Twitter
Anders Blirup Worm
@AndersBlirup
Some people call it farting in bed. I call it sheet music.
3:13 PM · Jul 23, 2019·Twitter for Android
 
Twitter
Jam Jammy Jam®
@jamlekjakababa
If your Monday has been filled with blues this is for you;
What do songwriters compose in bed?
Sheet music.
Then someone comes along and does a cover.
#stolen
10:35 AM · Sep 23, 2019·Twitter for Android
 
Twitter   
Alan Elwell
@elwell_alan
Why did the composer spend all his time in bed ?
Because he wrote sheet music.
2:48 AM · Jun 8, 2020·Twitter for iPhone
     
Twitter
Th3Jay
@Th3Jay2
Farting in bed is just sheet music
1:30 AM · Jul 31, 2020·Twitter for iPhone
   
Twitter
パララックス
@Parallax_
Farting in bed is just sheet music.
5:38 AM · Jul 31, 2020·Twitter for iPhone
 
Twitter
Ultimate Yang
@ultimate_yang
Late night inspiration: fart in bed and call it sheet music
4:16 AM · Aug 3, 2020·Twitter for iPhone
 
Twitter
Brent W. Peterson
@brentwpeterson
Did you hear about the composer who won’t get out of bed?
He writes a lot of sheet music. #brentjoke
10:15 AM · Sep 6, 2020·HubSpot
 
Twitter
James
@Jamesfishaa
You call it farting in bed
I call it sheet music
We are not the same
7:00 PM · Sep 18, 2020·Twitter for Android
 
Twitter
No Baldi stop
@spaghettioshoe
Farting in bed more like sheet music
5:25 PM · Oct 23, 2020·Twitter for iPhone
 
Twitter
Jimmy RevJim Olsen
@mybuddyjimmy
What do you call a fart in bed? Sheet music! #LaughoftheDay
7:13 PM · Nov 7, 2020 from Des Moines, IA·Twitter for iPhone