Hi there,
I am having a strange issue with my usb sound card and and Compute Module 4. I have enabled the USB on the CM4 IO motherboard, it detects the sound card but after a while it disappears. I test the same sound card with the same build on Rpi 4B without having any problem. Maybe is a bug on kernel or not fully supporting the usb drivers?
I need some help or some information.
The sound card is SMSL Sanskrit 10th mkII.
DietPI v7.9.3 64bit
On the DAC display it shows D-8 I do not know if it is an error on manual it does not saying anything about this. This d-8 happens only on CM4 and not on classic RPI 4.
Best regards and Merry Christmas . 
-----Edit------
When I manage to make it work after reinstalling the DAC to audio settings this happens:
First reboot working normal.
Second reboot not detected.
Third reboot working.
Fourth reboot not working.
In simple words to make it work you need to reboot twice. I have also a second DAC Khadas Toneboard it does the same thing. Maybe a bug? This is only on CM4 with IO board. Tested also the 32bit version of Dietpi do exactly the same thing.