Problems with connecting the I2S audio devices to JNano - logic conversion

I want to connect an i2s input and output to JNano (4 x Adafruit SPH0645 microphones + UDA1334 DAC) . Since the I2S4 suffers from the problem with the TXB0108 logic level converter, I decided to make an NGFF M2 key E card to use the I2S3. The I2S3 logic is 1.8 V, therefore, I have used widely known high speed optocouplers, HCPL 2631 dual, to shift it to 3V3. And the audio output through UDA1334 DAC works! The key to success is the length of the connections, they need to be as short as possible.
However, I have problems with the microphone input. The same optocoupler has been used, but in reverse direction. But does the I2S data-in pin require a pull up or pull down resistor? It seems the pull-up configuration is not working, the signal at the pin has only a few millivolts range…

Seems pretty complex. Why not try a mic-hat, like this

No pu/pd on the pin. As it is an input signal for nano, seems more like a problem of device. Is it possible to observe the signal without nano?

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Thank you guys for help. I discovered that the output current was not enough. Solved by replacing the optocoupler with a simple resistor - red led pair, in order to convert from 3V3 to 1V8. Now the microphones work as well.