well tehnicly under berock is the void but it could be possible you hear ghasts cuz the top of the nether has a bedrock althought i broke the top of the nether bedrock(with the ultimate pick or something) and i was on a infinte bedrock roof i wass all like WTF but it is possible tha tthe nethers bedrock is conectd to the overworlds so the sounds are from the nether :3 if that happened t ome i would shit my pants quit minecraft jion in 10 minutes,jump like crazy for the ladder and ask "WTF was that?!" xD
