John,
You can load in a midi file,(or record one in the SONG CREATOR), then you can go to the USB recorder, press RECORD, press PLAY on the SONG recorder and your MIDI is recorded as audio on a flashdrive.
You can also plug in a microphone, or another keyboard, and whilst the midi is being recorded, you can add a vocal or another instrument part to the audio.
Some people record background vocals along with the midi...then they can sing along with the resulting audio and have more complex harmonies(like do-wops) not possible with using the harmonizer alone.
It is easier to do than explain...you can find the S900 manual on line if you wish to explore further.
Ian
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.