Check your options>Audio. If MC is coming out of your main monitors the soft synths should too. If you arent' using ASIO, WDM might have both interfaces being used simulanteously, and your midi could be using that. Unlikely, but could happen.
Also check your mixer hardware outputs. I don't know how MC shows it, but in SONAR you can see the mixer tracks, buses and hardware outputs. Somewhere, some tracks could be routed directly to the hardware outputs (your Dell soundcard). Change the track to your main output interface.
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