Saint Ajora: Why is it you hear Monique sing and Elle sing, but Gilbert playing his [lute] does not change the background tune?

It was a quiet day in Geo, or at least as quiet as it got in such a large city. Only the background bustle of comings and goings outside and the idle chatter of students on their way home were heard in the Sorry, Carl Cafe. Suddenly, the calm air was rent asunder by a piercing cry:
"OH, my love! We were meant to be together, baby! Come with me to a faraway land!"
Everyone's favorite (and only, thankfully) lovestruck centaur was accosting Leandra, one of the boardmembers of the Geo Magic Association, a powerful sorceress in her own right.
"Look, buddy, I don't know what your problem is, but I'm trying to enjoy a drink here, do you mind?"
Gilbert, dense as ever promptly responded "Then let us drink together, for years of happiness and love to come!" With that, he unslung his lute and began to play, much to the chagrin of everyone in the cafe. To their further horror, he began to sing, as well.
"Ohh, baby! You know we were meant to fly! I love you so much that I could cry... And if we should ever part... I'll just try again, from the start!"
Leandra's mood, quickly gone as dark as her favorite Shade flute, prompted immediate action. She mentally cycled through a list of incantations, ANYTHING just to make this oaf shut up. She quickly realized that anything nasty, such at turning him into a toad, would only result in him hounding her until she changed him back. Then, an idea struck her. What if she made his music sound like nothing to everyone else, but he could still hear it fine? He'd ever catch on. She smiled slyly at him, already gathering mana for the effort. Gilbert, as always, totally misinterpreted this.
"Ohh, sweetness! My singing has won you over! That look! The angelic face of rapture! I can't resist! Give your talented, trail-weary troubador a kiss!"
"On one condition," she said. "Allow me to examine the instrument which has produced such fine music for me."
Gilbert was ecstatic. "Anything, my love! For you, I would uproot Gaeus the Earth!"
Leandra took the lute and enchanted it, so that only the player would hear the wretched melodies that it belched forth. "Now, to get rid of the fool," thought Leandra. Her train of thought was rudely interrupted by a face, grinning inches from hers.
"Your minstrel awaits his reward! Here's to TRUE LOVE, baby!"
Leandra sighed. She promised, there was no way to get out of it. She steeled herself, then planted a determined pack on Gilbert's cheek. The response was immediate. His eyes widened, his knees buckled, and the whole bard went tumbling to the floor in a heap.
"Figures. For all his effort, he's never even been smooched. Probably BECAUSE all his effort." She left before he had the chance to recover.
So, Gilbert still travels on his endless quest for someone who would rather kiss him than pop him on the kisser, blissfully unaware of his curse.