According to the opinions that one may not listen to a woman's recording:
I've been playing around with some music recording programs, particularly the functions that modulate the voice and apply filters to make it sound different. This got me thinking: what if a man sang and applied these filters to make his voice sound like a woman's? What about the opposite, a woman singing and modulating her voice to sound like a man's? Is there a kol ishah issue in either case? That is, do we go after who's singing, or what it sounds like on the recording?