What a project.
Chang (hua) mun 2 (ช่าง (หัว) มัน) means “let it be that way or who cares?”
Normally, Thai people say chang mun (who cares?, never mind) or chang mun ther.
It is one of Thai emotional expressions (read more about Thai people in Thais are so emotional). When they feel, “Okay, I’ll face it whatever it will be.” Or “Forget about it, mun.” and it is also meant nicely reply to an appreciation, a regret expression, or even a decline that Never mind.
Chang hua mun is for more intimately speaking than Mai pen rai (rawk) (ไม่เป็นไร), since it is spoken in a family or close friends. It can also be Chang mun ther.