Khiltā is second-person masculine singular past, “you imagined,” not a first-person verb. Its root is kh-y-l and it conveys supposition. Ṣawtahā is the sound being imagined; tajāwub aẓʾār is the comparable pattern of answering calls.
Idhā rajjaʿat specifies the moment when the singer modulates her voice. The simile is therefore filtered through the listener’s perception, not presented as an objective statement that one sound literally is another.