I also installed an aftermarket carplay solution in my 2015 Macan, and quickly realized the issue that OP is describing...
This module has a direct USB connection, which I routed to my center console. After few weeks trying to tweek the audio with it, I decided not to use it as an input anymore, nor the wireless AUX input for audio, as it routes the audio through the Auxiliary input through box, and in turn, degrades audio quality. The issue with audio quality became very apparent when I started to do a speaker upgrade.
The Macan have a digital optical communication system for audio (MOST25), and is a bad idea to have any DAC in between your source and this system. To maintain the sound as pure as possible, digital signal should be maintained from source, all the way to the amplifier.
So, what I do is, I connect my phone to the PCM using the OEM USB connection that goes directly to it, to ensure the highest quality of sound by keeping the digital signal intact and I'm able to connect my phone to the carplay module wireless via WIFI, which allow me to see CarPlay in the screen. The audio digital signal never goes through the carplay module, only the information needed to project the carplay screen. The only change you'll have to make is to activate the wireless carplay in the module, and connect the phone via the USB PCM iPod input. Instead of selecting AUX in the PCM, select iPod.
There are some minor compromises by doing this. For example, once carplay is connected via wireless, some map/gps applications like Waze and Apple Maps, wants to route their audio through AUX connection, not through PCM, hence there is no audio from those applications when set to PCM. Google Maps however, have the capability to choose where do I send the audio (via AUX or PCM), and this works well when the right option is selected.
In addition, there is a connection sequence that should be followed. Once you start the car, you must allow carplay wireless to connect first (you'll see the carplay screen), then you connect the PCM USB cable, go back to the PCM screen and select iPod in the PCM and then press the NAVI button the required number of times to bring up the Carplay screen again. It may be a bit of a hassle in the beginning, but now, I do this without even thinking.
Also, you should deactivate the PCM AUX Bluetooth option, it only interferes with that carplay module operation, and always connect via PCM iPod USB to ensure best quality.
To me, this is the best compromise, as I’m not willing to degrade Music audio quality. I normally use my car to commute, and actually prefer a silent gps application. When I go somewhere else, I normally use Google Maps and have Waze running in the background.
My audio system upgrade history (including carplay) here:
My 2015 Porsche Macan Audio Upgrade Log
