Are you getting sound from sources other than FL6? If so, then be sure that your Audio Settings are correct (Options > Audio settings). Be sure you have the correct audio driver selected in the drop-down, and is "Auto-Close Device" checked? Also, are you running any other production software at the same time? If so, try shutting everything down and just open FL. Rewire seems to bug out on me sometimes and mutes FL. Also, when you try to play your FL project, does it look like it's playing, just muted, or is it frozen?