When is it necessary to use "da"?
Why not just pli volontuloj?
Because "pli" is not an adjective -- you could say pliajn voluntulojn, which would mean something like "additional volunteers" however rather than "more".
I hear "benonas" with no "z" sound.
I really, really, really wish Esperanto had a slow audio option like all the other languages.
I hear "belonus". No "z" sound.
Trinku akvon, rakontanto! Dolorigas min aŭdi vin lukti.