If you're talking about acoustic instruments, I'd say in the right hands a violin or a cello played in the mid to higher registers are good candidates. If you're talking about electronic instruments, most anything is possible. For analog electroni... read more
As an alternative to the Theremin, I’d say that certain types of Organs can kind of ressemble the female voice. If electronic instruments are included, I’d say that many pads and synths have an even more similare tone to that of the female voice. read more
Most men with good singing voices can reproduce the sound of a cello or a trombone pretty easily. And the other way around, I suppose a barry sax (it even sounds like a person's name) could come close to a human voice, as could harmonics on a cello, or a basooon or tuba. read more