If you hear it then 2 things:
It's audio outputs are hooked up to your monitor system so turn it down or off. Yes the volume control would be logical. As Mr Bone say's most people turn off "local" or you will hear it's sound when played.
It is receiving MIDI data from somewhere, that means you have the track set to send midi data to either the USB or MIDI output port. disable this. Check your tracks output is set to the soft synth only and that the soft synths are set to master only.
3.) MIDI OUT is set to the Stereo OUT on my F'wire interface. I don't think so, Audio outputs are not an option for midi output. It's either a midi port, USB or to the MS wavesynth thing.