This query should have totally been bumped in the "
silly questions" locale... but I can dig it.
Also, I have your answer. If you want to refer to the Japanese manga/anime example, it is pronounced is a soft 'nee-ah', and as often seen spelled as 'nyah', but yours is close enough. Almost forgot to tell you: it is also almost always used in conjuction with the "eye-pull-down" (you either know what I mean or not) or an exaggerated bend over and slap on the behind.
Referring to its practice in North America, it caught on some decades ago, and I always thought of it as the condensed word-play on 'nanny-nanny boo-boo'. Sounds just the same: a 'nyah' (and it is meant to sound like a cats' mew... I couldn't tell you why; perhaps ask why 'catgirls' are so big in Japan).