I'm wondering if a television drama might have inspired the "When I Can't Play" song.
Back in the early 70's there was a TV series called "The Bold Ones: The New Doctors". One of the episodes included a pop musician with a neurological disorder that would eventually affect her hands and throat. But rather than being depressed, she was determined to enjoy life and music as much as possible. One of her lines was something close to, "When I can't play, I'll sing, and when I can't sing, I'll listen."
This line affected me enough to remain in memory after all these years (and I can't remember a single other thing about the series). Perhaps it sparked a song in Joan MacIsaac?