Always used balanced connections between your audio interface and the mixer. Also, try to use balanced connectors to your monitors. Balanced lines reduce the likelihood of interference as the cables have an additional level of shielding over a 2 conductor unbalanced link.
Common balanced connectors are: 1/4" or 1/8" TRS (tip, ring and sleeve) jacks or XLR
The problem is that your Audigy sound card possibly doesn't support balanced connections
ps: digital solves a lot of problem as above but I doubt you have a digital mixer
