Different voices, different volumes

Hi,
I'd like to set up a multi-voice score so that one voice plays back at a louder volume than the others. Probably I've missed something, but I haven't found a way to attach different dynamics to different voices.

Example: As an exercise I'm scoring Satie's Gymnopedie #1. Probably you know it: there's a simple melody playing over a background of chords. When I play it back, the melody gets overwhelmed by the chords, and I'd like to set its volume higher relative to the chords.

I hope this makes sense.
Thanks,
John B.