A viola joke is:
   
Q: How do you make a violin sound like a viola?
A: Play in the low register with a lot of wrong notes.
 
The joke was posted on the newsgroup rec.humor on July 8, 1992.
 
“How do you make a violin sound like a viola?”/“Sit in the back and don’t play” is the viola joke with a different punchline.
   
 
Google Groups: rec.humor
Jokes - Music
Rich Ormerod
7/8/92
(...)
Q: How do you make a violin sound like a viola?
A: Play in the low register with a LOT of wrong notes.
   
Google Groups: rec.humor
Canonical List Of Music Humor 2/3
Two More Ron’s Don’t Make A Wright
8/3/94
(...)
How do you make a violin sound like a viola?
Sit in the back and don’t play.
  or
Play in the low register with a lot of wrong notes.
 
Google Groups: rec.humor
VIOLA JOKES
Fil Feit
10/6/94
(...)
Q:  How do you make a violin sound like a viola?
A1: Sit in the back and don’t play.
A2: Play in the low register with a LOT of wrong notes.
     
Twitter
Gerald Maio‏
@GeraldMaio
Q: How do you make a violin sound like a viola?A: Play in the low register with a lot of wrong notes.
1:37 AM - 6 Dec 2009
 
Google Books
The Musician’s Joke Book
By Jim Green
London, UK: Omnibus Press
2010
Pg. ?:
Q: How do you make a violin sound like a viola?
A: Sit in the back and don’t play.
 
Q: ...And another way to make a violin sound like a viola?
A: Play in the low register with a lot of wrong notes.
     
Google Books
Sensible Nonsense.. Frank’s Comic Pack:
Humour Pack Series
By Frank Paul
BookBaby
2014
Pg. ?:
Q: How do you make a violin sound like a viola? A: Play in the low register with a lot of wrong notes.