Use the top four slots for your two video cards. Then use the bottom or the 2nd from the bottom for your sound card.
If not, it appears you bought the wrong case, because a 7 I/O slot case with a MB having Tri-SLI capability, is stretching it thin. This seems like something that should have been caught before any of the stuff was purchased.
I wouldn't even mod the case if it won't work, sell it and get something better suited to your MB.