“Aiyo!” or “Aiya!” ?
I’ve heard both, the former largely from Cantonese speakers, the latter from Mandarin speakers. Not as though I have a large enough sample, though, to say there’s a pattern (and, I think I’ve heard the same people sometimes use both).
i think if they are surprised or in pain they get blurp out whatever?
I lived near Beijing and all I ever heard was “Aiya”.
Both ‘aiyo’ and ‘aiya’ are acceptable in Cantonese. Actually, the second syllable is often softened to something closer to a schwa nowadays.
Of course, when a Cantonese speaker is REALLY surprised/vexed/pained, more likely than not that will also be accompanied by swearing. ‘Aiya, puk gaai lo, diu!’, for example. 
I picked this up when I spent a year working as a dealer in a casino. My expressions range from “aiya” “aiyo” and every now and than “hiyoooooooooh”
Amy from Futurama says “Aiya” followed by what sounds like it could be profanity.
Ed McMahon Lives!
I am not native Chinese but speak Cantonese and lived in Hong Kong for a while. When I read the thread title, without reading the post, I immedately thought “Ai Yaa!” The “Yaa” that I heard in my head is schwaa-ish. Kind of in between the “AW” in "Jaw"and the “O” in “On” sound.
Mandarin speaker here. “aiya” is general surprise or alarm. “aiyo” I would say is more of the “tsk tsk” or “oh no, not again” variety of exclamation.
Another Mandarin speaker here (Taiwan). I hear both a lot. What robardin says makes sense, but I never paid close enough attention to be sure.
Aiyo is more common in Shanghai than Aiya.
A Chinese immigrant I know says “Shit!”
My father-in-law says “aiya,” with the last vowel either a schwa or, if he’s really surprised, drawn way out.
–Cliffy