OK, here's a step by step:
1. Select the midi file you wish to revoice.
2. Select Digital Recording, then select Song Creator
3. Tab to Channel
4. Using the "B" button on the upper left corner of the display, tab down to Setup and place checkmarks in all the appropriate boxes by using the #8 button at the bottom right corner of the display.
5. Press the Mixing Console Button and using the "F" button in the upper right corner of the display, turn on the Song Auto-revoice button.
6. Press the Exit Button once. This will take you back to the Song Creator: Service screen.
7. Press the Execute button on the lower left side of the display.
8. Press the Save button on the lower right side of the display, which will open another page allowing you to press the bottom of the #6 button for the final save.
9. Name the new song with another name so you will not overwrite the original midi file, then press OK.
Now, this is the procedure for the PSR-3000, and there may be some variations on the Tyros-4, but basically it's the same.