I just read thet you wanted him to shout :P
Replace:
selfSay(random_texts[math.random(1, #random_texts)])
with:
selfSay(random_texts[math.random(1, #random_texts)], 0, TALKTYPE_YELL)
0 - player id 0 - doesn't exist, npc will speak on Default window
TALKTYPE_YELL - yell :)