To me it sounds like you need a new dealer. Get a second opinion if this dealer won't work with you, call up GM's customer service and create an account so they can keep track of what's going on. They have the power to call your dealer to work with you. An audio system has nothing to do with powertrain warranty. The audio system has everything to do with bumper to bumper. The engine failure is covered in the powertrain warranty so that's your argument for that comment. As for the SLP axleback and intake causing an issue, tell him to prove it to you that it is an issue otherwise he's gonna hear it from your lawyer. There are law in place that say you need to prove that the mod caused the issue to void the warranty.