I junked muddy-sounding PCM 2 within a few months of buying an 04 CS with standard speakers. Highly unimpressed with standard audio.

Replaced with a Kenwood DNX4210 DAB unit (UK) and sound quality now markedly better. I was planning to rip out the speakers the following week but that can wait a while as this is more than bearable.

Next got to drill the roof for decent DAB/digital radio antenna. Will fit some Philips DAP amps with Kef 8" subs and B&W/Kef speakers in due course.

Bose audio systems
 

I have had two cars with Bose audio "upgraded" sound systems. They were not awful, but it is like I have heard it said, "if it's Bose, it has no highs or lows" I have a tendency to agree. The system performed ok, very flat, just not as good as I think The Bose company thinks they sound. I optioned my MY13 Cayenne with the Burmester audio option. While I do not consider myself an audio expert, I have to say if you are looking for a incredible sound system for a vehicle, this is it. Though a comparable quality replacement system might be able to be installed at a lower cost, I am very pleased I spend the extra to have this in my Cayenne. I believe this is the gold standard for an audio experience in a vehicle. I agree with others that the Bose is not all it is cut out to be. Just my opinion.

Car play
 

I bought CarPlay to cayenne 2012 and the sound improvement was insane compared to what it sounded before.

Bose system definitely leaves something to be desired, but with an aftermarket head unit and a JL box in the trunk, it sounds okay. I'm still on the fence whether I'm keeping her ('06 TTS) until she's antiquey but at 15yrs, how much longer could it really be before the speakers start to disintegrate anyway? Part of me isn't looking forward to dropping a few grand on a new system but part of me is.