That's what I though, but it doesn't. I haven't altered the ammo.fpi at all and all sounds are in the audiobank. I've got round it making a copy and adding "sound=audiobank\misc\talkping.wav".
;Artificial Intelligence Script
;Header
desc = Ammo Pickup
;Triggers
:state=0:hudreset,hudx=50,hudy=90,hudimagefine=gamecore\text\pickedupammo.tga,hudname=ammoprompt,hudhide=1,hudmake=display,state=10
:state=10,plrdistwithin=40,cantake:state=1,playertake,coloff,sound=audiobank\misc\talkping.wav,plrsound=audiobank\items\pickup.wav,hudshow=ammoprompt,hudfadeout=ammoprompt
;End of Script