Hi Adam, welcome to Cayenne Forums. AFAIK - there are no separate wiring diagrams for the MOST system. It's an optical fiber system... error codes can be a bit deceiving since they aren't dated - and if someone disconnected some component of the system and turned the system on, an error code would be stored. It would have to be manually erased, which probably wasn't done.
I'd be looking at the wiring for the door/speakers. There is a connector on the front edge of the door that can be disconnected, cleaned and reconnected. I'd suggest that be done first. The connections in the door are for the large speaker and the midrange speaker. To check those you'll have to remove the door card - something I unfortunately have some experience with - it's actually quite a chore to get it off, and you will break some of the retaining clips.
Depending on the level of DIY you anticipate doing on the Cayenne - it might be worthwhile to get an AllDataDIY subscription for the car. That's pretty much the factory manual on-line. The level of detail varies depending on what the system is they're covering, but it does have wiring diagrams which can be quite useful - and now has them in color and actually useable.
They are offering a discount - - and I have the info stored on another forum I run - Planet-9.com (a lot broader coverage of different Porsche models.) - let me see if I can find it..
I take no credit for finding this, but I came across this deal at my favorite deal site. Here is the link to the post: 3-Year ALLDATAdiy Subscription (One Vehicle) $50
Basically, you can get a 3-year sub, for one vehicle, with the coupon code DIY3YRMATCH.
That's actually quite a good deal.
Good luck, let us know how it works out.