AFAIK there are no free fully implemented standalone JSAPI compliant speech recognizer. I am afraid you will have to use Sphinx.
Moreover, "retrieve a sentence" is not a simple task at all. There is actually a lot to do before you even start recognizing the sentence. Look at the guides for sphinx:
http://cmusphinx.sourceforge.net/wiki/
http://cmusphinx.sourceforge.net/sphinx4/doc/ProgrammersGuide.html