!N::Send {Volume_Down}
!B::Send {Volume_Up}
! is the modifier for alt.
Just for the sake of it, heres what I use for the other media keys:
;next song
+Pause::Send {Media_Next}
return
;previous song
+PrintScreen::Send {Media_Prev}
return
;play/pause
+ScrollLock::Send {Media_Play_Pause}
return
+ is the symbol for shift, ^ for control. I think @ is for windows, but I'm not sure, because I've never used it.