My first two thoughts are: make sure the batteries are fully charged, low batteries will cause the sound to die out. And with the V4 soundboard there's a mute setting, the last out of 8 sound fonts - make sure it's not set to the mute font making you think the sound doesn't work.
Give those a try and let us know any results
EDIT: GENTLY have a look at your battery pack and do an inspection for any loose wiring, this can happen on the odd occasion.