You could try "nee-naw" instead of nee-naa, perhaps. Wouldn't recommend going with the wee-waw one. Or just have a "weeee" sound affect that gets smaller the further away the ambulance is.
I'm hugely impressed that you have a link immediately to hand for this CharleyHorse!
My choice would've been to just use a EEEEEEEEEEEEEEEEEEEEEEEEEEE tapering off into the sunset: leaving out the consonants avoids the distracting cliches and you know how it's supposed to sound anyway.
The reason that I prefer Ironscarf's version is that it seems both more mechanical in nature and hysterical, which is what an ambulance siren brings to my mind. But it's all down to personal tastes.
you could always just be hilariously sarcastic and just do "SIRENSIRENSIRENSIREN!"
"Kung Fu Komix IS...hardcore martial art action all the way. 8/10" -Harkovast
"Kung Fu Komix is that rare comic that is made with heart and love of the medium, and it delivers" -Zenstrive
"Kung Fu Komix is...so awesome" -threeeyeswurm
"Kung Fu Komix is..told with all the stupid exuberance of the genre it parodies" -The Real Macabre