I did some tests and can now confirm that Deafening Whisper will only trigger Nazarec Sin's ability, Abyssal Extractors, from a direct impact kill from the grenade launcher itself. For whatever reason, the void wave that the grenade launcher creates[i] does not trigger abyssal extractors[/i]. If you have wellspring on the grenade launcher, you will still gain ability recovery from that perk, but you will only get ability energy from Nazarec Sin on direct impact kills.