I don't know if you've got the PDF version of the manual, but if not, I highly recommend going to Yamaha's web site and downloading it, because it's much easier to look for information in a PDF file than a printed manual by using the Find (CTRL-F) command to search for occurrences of the word, phrase, or combination of characters that you're looking for.
See the section called "Revoicing" which begins on page 78. It tells you how to change the parameters used for the voices you've selected for the R1, R2, and L parts or voices.
On the newer PSR-S, Tyros, and Genos models I believe these functions are referred to as the "mixer," which is why I mentioned the mixer screen.
On the PSR-E models (which are the kind that I own) there is no "mixer screen," so one must enter the function menu and navigate through the individual functions to find the parameters one wishes to change.
Anyway, the MASTER VOLUME knob controls the amplitude of the sound coming from the built-in speakers, or being sent to the headphones or audio output jack, but turning up the volume to hear the L voice better won't do any good if everything else gets louder, too!
So the REVOICE button lets you display the parameter settings for the R1, R2, and L voices-- as well as the song or accompaniment tracks-- so you can change those settings as desired, which lets you create a better balance between the volumes of the various voices and tracks. Then, once you've got a more pleasing balance, use the MASTER VOLUME to adjust the overall volume.
