Guitar Quote, not "Violin"
The "Violin" quote that I have been searching for turns out to be a Guitar quote, and it was from Hoekman, not Cockburn:
"Playing the guitar badly is vary hard, Playing guitar well is very easy." (Designing the Moment, Hoekman, 2008, p224)
Sadly, the "veteran master" that Hoekman attributes this to is unnamed.