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Assuming you have the performance exhaust option...You can change the valves by switching the mode on the center console. Tour, eco, weather are quiet. Sport and track open the valves so more sound. There are also adjustments in the settings on your center screen.
Yea, the settings can override the mode selector ****. With all of those, the valves can still be closed in some cases. Which is why some pull the fuse (can be different depending on year, your profile does not say what C7 you have).
Pulling the fuse keeps them open all the time. If you want to be able to control it from the car though, you'll need to wire in a switch or purchase a Mild2Wild (M2W) module that will allow you to control it.
Search the forums for NPP, you should find plenty of info if you want more details.