Its been 2 yrs ago that I fiddled with this board with ESP ADF and i was able to switch between mic and line inputs, both or individual channels, so far I traced this line in the code:
res |= WriteReg(ADC_SRC, 0x0408); // 0x2020 for mic input // 0x0408 for line input
If you want i can get the address of ADC_SRC, but, i found it on a data page on the AC101, i may still have the pdf?
Those values are for both channels, and I can't recall if you can enter 0x0420 or 0x2008 if that is how to select each channel, or it was a little more complicated than that, I only needed to flip both at once, but, this would flip between mic and line inputs on the board, Where in your code do you write to the registers of the ac101 if i knew which files, i could try it (although depends on my availability)

So far I modified the first file, the .c file in audio_driver/ac101 and it now defaults to line in on both channels

on line 168 res |= ac101_write_reg(ADC_SRC, 0x0420); // 0x0408); // 0x2020); 0x2020 both channels mic in 0x0408 both channels line in
0x0420 gave right mic in left line in

I had a hard time with the PCM1808 and the raspberry pi pico, but finally got it going. You can see my log of trouble here.

Why would you need to port anything ?
As far as I can see it's just a question of configuration...
What processor and Arduino Core version are you using ?

Maybe I misunderstood the situation. I didn't connected a pcm1808 yet, nor checked the code. Reading this thread I meant that this chip is not working with the audio tools, there is some problem with the clocks or wiring. So I searched for a working code, and found the above link. Maybe I'm wrong?
I succesfully tried some of your examples using the ESP32-A1S module, and have a pcm1808 chip as well, and about to try that one too.

You need to define the masterclock pin with pin_mck. But be careful, on the ESP32 there are only 2 pins that work for this purpuse and usually the gpio0 is used for this. I am not sure how important it is to use 32 bits for this to work...

Just a small clarification because your usage of board is a bit confusing: when you set is_master = true then the ESP32 microcontroller is the "master" and the I2S breakout board is the "slave". I usually prefer to use this because then you can select the sampling rate in your code and it is not predefined by the hardware.

In this case the master clock is generated by the ESP32 and you can use either pins 0, 1, or 3. (However avoid 1 because this is also the serial TX pin)

The default clock value is 256 * audio sampling rate if not specified otherwise.

The ESP32 only supports the master clock as output (and not as input).

Thanks for the clarification, I thought my writing would be understandable for everyone but maybe it's confusing.

The Problem is that I read only zeroes from the PCM1808's breakout board data_out. I suspect my board (an ESP32-Wrover-E, designed for a camera - a clone of the Freenove ESP32-WROVER Board) has a problem to generate the MCK. My approach is to replace the board as a next step.

In the rest of this reply I also wanted to clarify a bit from my end, hopefully better understandable than before:

Master Clock can only be used on pins 0, 1, or 3. (However avoid 1 because this is also the serial TX pin)

The (your) Audiologger also gives the warning and is known from the esp32 official source codes i2s.h: "Note that ESP32 supports setting MCK on GPIO0/GPIO1/GPIO3 only". Configuring the i2s port as master with pin 0 as mck should generate the master clock properly but it doesn't on my "fake" board. An esp32-s3 is capable of using any free GPIO pin for the MCK (indirect source), I will try that next.

The I2S PCM1808 breakout board is slave, and the ESP32 is master, with is_master = true**:

Good to clarify if that wasn't clear, to me it was. I understand that it's preferrable for more possibilities, and my new insight was that my I2S PCM1808 breakout board is also not capable by itself (without connecting a crystal osci) to generate a mck (see also), hence is_master = false does not work out of the box with this breakout board anyway - somehow I thought the breakout board would provide the master clock to itself if not provided, but that's not implemented in hardware on the/my board. Good to know that the ESP32 does not support master clock as input. So bascially the mck-pin is not used when is_master = false. Makes sence (we still have a sample rate), thanks.

The default clock value is 256 * audio sampling rate if not specified otherwise.

Now I also read the following: Please set the mclk_multiple to I2S_MCLK_MULTIPLE_384 while using 24 bits data width Otherwise the sample rate might be imprecise since the bclk division is not a integer(source). Since the PCM1808 is 24bits, I stick to 384fs now. I played around with the fixed_mclk a lot, and below a certain rate (something like 96fs) it doesn't even output zeroes anymore, which is at least a sign that the breakout board still responds. 384 always worked to at least output zeroes (512 and 256 too though, as they should).

I attached my code, which I suspect would run just fine on another board. Btw I can have some noise if I touch the data-pin, but it never seems to correlate to whatever is connected to the L_in / R_in pins (stereo cable) on the actual breakout board.
main.md

Thanks so much for your support again, I will report back asap whether it runs on a different board.

I don't know if you read my post above, but I was using a Wroom board and despite specifying the MCK to output on pin3 within the code, the Wroom board was actually outputting it on pin 1 (tx).

I'm curious to see if you can get that purple ADC to work, because I have one but was not successful (didn't try that hard). I was successful with the PCM1802 with some "mods"

GPIO 3 has always been the TX pin !

Okay then. I was specifying gpio3 in the code but it was actually outputting the mck signal on gpio1 (for the wroom-32)