Sometimes the SOC contains all the hardware, and even the codecs, but they are activated/deactivated during production (by programming the non-volatile memory in the chip) depending on which codecs have been licensed. This memory is not always accessible without specialist equipment. I don’t know whether this is the case here, and I really don’t know whether the audio quality issues are even related to the codec, or whether something else is going on.
Also: I’m in no way saying the issues cannot or should not be fixed with the current hardware, but for those interested there are some audio-related posts elsewhere on the forum:
USB-OTG external sound cards seem to work:
In terms of providing modules / extensions, if USB OTG devices work, the built-in USB port (under the back cover) will see more options after the next software upgrade. Discussion on USB specs is going on here (rather spread out throughout the topic):
So I would guess there is a possibility to integrate a USB DAC in a modified backcover - but I’m not a hardware expert, so reality may be different.
Finally, module suggestions are being collected in the topic below, but again, whether any of them will ever be realised is anyone’s guess.