This is the frame sync clock. It’s frequency should be same as the playback sample rate (48000 Hz in your case) in I2S mode.
I hope you are setting ‘x SPKLO MUX’ control to speaker.
I don’t know what problem it is. You can mention the DT binding you are using for passing the GPIO info and also mention to which pin it is connected on 40-pin header. Also is this essential for codec operation?
Looks like the clocks and frequencies are fine. You can attach the scope capture (with and without your card connected) as requested before, so that we can confirm.
I also see ak4637_hw_params() call to be happening. This confirms the audio routing is fine. So things look good from Tegra point of view.
You can confirm the codec settings with codec vendor.