I am looking for a plugin that can help convert my text into an audio.
For Example
I want the app to spell out Hello World while the input provided is Hello World in written english.
Not that I've seen. For a small number of words it wouldn't be too difficult but if you wanted to support the whole of English (never mind other languages) it would be a huge undertaking. Having a recording of every possible word wouldn't be realistic so you'd need some sort of speech engine to generate the audio and that's well beyond what's realistic with client-side JavaScript. Might be possible with Flash but more likely it'd need to be done on the server.
Text Speaker can convert your text to MP3's you can use in your app. It has natural sounding voices with a selection of male, female, English, British, French and Spanish voices: http://www.deskshare.com/text-to-speech-software.aspx