It's in the manual, at least the nav system manual.
From page 4-12 audio system of the '13 Leaf nav manual:
"[Speed Sensitive Vol.]:
The sound volume is increased based on the
vehicle speed. Touch [+] or [-] to adjust the
effect level from 0 (OFF) to 3."
And yes, I have nav and use the above.
On page 4-36 of the owner's manual (for cars with no nav, that is the S trim):
"Speed Sens Vol.
To change the Speed Sensitive Volume
(Speed Sens Vol.) level from off to 5, turn the
TUNE·SCROLL control knob."
'13 Leaf SV w/premium package (owned)
'13 Leaf SV w/QC + LED & premium packages (lease over, car returned)
Please don't PM me with Leaf questions. Just post in the topic that seems most appropriate.