The story goes that Glenn Gould hummed and sang along while he wove his magic at the piano because his mother — his first piano teacher — had taught him to. The idea was ostensibly to develop a stronger connection with the melody by immersing himself in it.
Glenn Gould could not stop singing, neither while on stage nor in the studio. Listen to a recording, any recording. Listen hard. Somewhere behind the piano is the sound of one man humming, usually adding an additional layer of counterpoint to the already-polyphonic suites of J.S. Bach.
A recent re-release of his 1955 recording of the Goldberg Variations includes audio of some of the studio outtakes. "Did any singing get on that one?" he asks the recording engineers after one take. He told one interviewer that he was aware of the issue, that he didn't do it on purpose, and that he didn't even care for it, but that he couldn't help it. It just was. Thanks to the miracle of recording technology, it still is.
"Cantabile" can be translated from the original Italian as "song-like" or "in a singing manner." As a musical term, it directs the musician to interpret an instrumental passage as though it were being sung. Not only does the melody really have to stand out against the accompaniment, it should ebb and flow as it would if a person were singing it.
"Maybe you should sing while you play," my piano teacher once said to me after I'd made it through my latest piece without any technical issues but, apparently, with some flat delivery. "Like Glenn Gould used to do. It's amazing, what we can do with our voices." She played the melody line of my piece again, singing along this time; her voice swelled and receded as I'd expect a singer's to do were it an aria instead of a piano sonatina, and it all made sense. I've played it that way ever since.
Glenn Gould's mother was on to something.