This is the meaning of prolonging tones.When you see a beautiful girl,you will make such a sigh"Wow~~". When you are hurt,you will say "Ouch~~".
Like this,HAha~~,do you understand?
In Australia, and probably the US, it's more common to add more letters - like Wooooooowww and Hahahahaha *snort* hahaaaaaaaa.
(In rare cases, unusual sounds or physical events are put in asterisks like the *snort* above.)
Or, perhaps more in line with your meaning, we'll append ! to words, such as Ouch!!! (The more ! the more you mean it, with limits of course.)
Capital letters are also used to imply that we're shouting or at least being loud. FAAAAAAArk.
So if I was saying thank you, I'd go Thank You!! - but I wouldn't say THANK YOU because that might sound like I was being rude.
(Emoticons are pretty standard ;-) )
Edit - since this forum is so international it'll be interesting to see how English changes over time