If you mean just the dialog, you'd do better starting from the 5.1 audio as a source. The centre channel should be dialog only.
If you want to include the music, or if you only have the audio as stereo, then it's not possible.
You cannot unbake a cake.
There is now the very good yet very expensive Izotope RX which has a "Music Rebalance" plugin ...
If you're talking about the MIDI FX plug-in, then here is a simple tutorial that describes the process:
Just drag the MIDI Pattern to a MIDI/Instrument track.
I own Omnisphere and find it to be incredible musical and flexible. And you could literally spend eternity experiencing every single patch available. My biggest gripe with it is that you cannot upload your own sounds (unlike Kontakt, Alchemy, and other sampler/synths). I happen to love the granular effects you can get in Omnisphere and I wish so often that I ...