Here is a first attempt at converting the audio clip to MIDI with Sonar, then playing the file back on a MIDI instrument. Used a lead guitar for the first voice part and then acoustic piano for the whistled part. Hope this is close?
https://clyp.it/jtvw0tfk The pitch detection was initially thrown off by frequencies outside of the main vocal phrases, so I used the LP-MB compressor to sharpen the focus on the strongest frequency bands in the melody. Then the conversion seemed to work fine. The quality of the microphone and the clarity of the signal would seem to be the biggest factor in getting a good voice to MIDI conversion. Maybe the kazoo is not such a bad idea?
Had to split the MIDI file into two tracks, one for the first part, and another for the second. Then used the MIDI transpose process to change octaves to those that matched the instruments for MIDI playback. The only effects used for playback were already in the instrument programs. All processing was in the box.