More information. What did you have before the Steinberg card?
However, I will hazard a guess from reading between the lines. I'm guessing you formerly had a generic sound card that had a MIDI synth on it. Typically, good recording soundcards do not have MIDI synths on them, so switching to the Steinberg would eliminate that device from your system. There is nothing to respond to MIDI messages and therefore no output.
You need either a softsynth (I don't know which types n-Track might support) or an external synth with the MIDI messages routed to it via a MIDI interface to get any sound.
You do know that MIDI messages contain no audio information -- no inherent sound -- don't you? That is, that they merely tell a MIDI synth what to play and when?