Internal warning noises dont work.

The buzzer or noise warning devices that make noise are usually pieze type device. Meaning they are very tiny and are seldom shown in wiring diagrams. They can also be integrated into sensors and I have seen a case where the radio speakers were uitilized. Piexo speakers are capable or emitting more than one noise, not necassarily at the same time as most wranings that sound different alternate.
The most likely scenario is that there is a pieze speaker that makes all the warning sound noises and it has failed. The service amual I have does not have the warning indicator shown and it might be for reasons that the manufaturer does not release that information for proprietary reasons. That is why dealer Service manuals cost several hundred dollars. You would have to call the parts department and ask if the buzzer, or whatever they call it, is a seperate part that you can order.
That is going to be the best way to get the information. You can also ask them where it is located and if it looks like it is hard to access or if there are a lot of parts to remove to get to it. If the buzzer is integrated into a part, which it could be part of the Security System as it has control of almost every sytem that has a buzzer that warns of said situations, the parts department at the dealership will not be able to tell you if it is in the Security control somewhere. If that happens ask to talk to the service manager. If that gets you no where, Porsche has a phone number you can call for issues. I would try to get another number from the service advisor that may not be listed in the owner's manual that will get you an answer that will be hlpfull as the issue number found in owner's manuals are for issues that are not down to that level of detail. So, demand a true Tech Help phone number becasue if a dealershipp can't answer you will a general help line be any different. The Manager will probably understand it is just not a number he is supposed to give out and should offer to call for you, especially if it is the dealer you purchased it from.
Also ask the Service Manager to show you all the, "Service Buletins". The are like recalls but are not dis closed to public unless asked for. You may find a bulletin related to your issue and then you can argue that it should be repaired for free or at a discount.
I hope that information helps.