Sakura cannot sing well. Quite the opposite, in fact—her voice wanders so horribly off-key that deaf men complain about it whenever she so much as attempts to hum the simplest tune. She is also quite unfortunately tone-deaf, and so fancies herself a wonderful singer whose voice could make her a star sensation if that had been the path she had chosen.
Naruto does not even think to tell her the truth because he is hopelessly, desperately, head-over-heels in love with her, and what is love but conveniently 'forgetting' about the other's faults? Even so, he can't quite hide his wince as Sakura hits what should have been a low note and yet still could have shattered glass; he is terribly fortunate that Sakura isn't paying the least bit of attention to him, lost in a mind-numbingly bad serenade of her own creation.