test for the presence of sounds used by eyelink toolbox
USAGE: err=testeyelinksounds(el)
el: if not supplied, we use 'initeyelinkdefaults'
to open a structure with eyelink default values
returns 0 when no errors, -1 if one or more are missing
Psychtoolbox/PsychHardware/EyelinkToolbox/EyelinkOneLiners/testeyelinksounds.m