I was able to squeeze out valid I2S signals of the chip without using the driver, just by direct register manipulation. However, I couldn't get the SLC interrupts working which is exactly the problem with the RTOS driver as well.
Unfortunately I couldn't find any information on how to enable SLC interrupts. The only description out there is a header file (actually two, slc_struct.h and slc_register.h, but they contain the same information in different formats) without any instructions on how to make the interrupts work. The most detailed hardware document I found provides no details either.
Anybody knows how to do this? Can somebody from @espressif comment on this?