Please find answers to your questions below:
1. Is UAC3 enabled by default (said master codes)?
Yes. UAC3 support is available in kernel by default. But, you need UAC3-compliant audio device to get this feature.
2. If I would like to make it disabled, what is the suggested way?
2.1 from something like menuconfig to NOT build the features from Kernel?
There is no menuconfig to disable this feature.
2.2 after booting into Celadon, any entry to disable this feature?
You can switch to backward-compatible UAC1/UAC2 configuration by writing 1 into bConfigurationValue sysfs.
echo 1 > /sys/bus/usb/devices/<n-n>bConfigurationValue
where n-n is the hub number and port number where the USB device is enumerated. You can get this value from dmesg.
And you will need to change the file permission for this sysfs to write into it,
Is your audio device having more than one configuration? In that case, the UAC3 patch caused an issue which makes the last configuration get selected for non-UAC3 compliant device.
You can cherry-pick this patch to get that fixed. This patch is available upstream:
Please let us know if you have any more questions.