I'm Hearing Audible Alerts For Charts I Don't Have Open

Don’t worry, you’re not going crazy and neither is NinjaTrader. You just have a different workspace open with those charts in it.

You can switch to that workspace by navigating to the following from the NinjaTrader Control Center: File > Workspaces > [Workspace name], then close the workspace by clicking File > Workspaces > Close Workspace.