First of all: the usage of the “ossc-0.78-aud” firmware is mandatory. Older firmware versions don’t have support for the PCM1862 and therefore won’t work. If you are suspecting flaws in your own firmware build: a definitely working “ossc-0.78-aud.jic” is available from ‘borti4938’, the creator of the OSSC 1.5 audio addon board. Just take a look at his GitHub repository:
If this still doesn’t solve your audio problem: i’ve build a couple of 1.6 OSSCs over the last months – 3 of ’em had no audio output. I suspected the ECS-3X9X 24.576 MHz quartz crystal… and was right. On all three boards i had accidentally created a small solder bridge between the two pins of the quartz. Sadly somehow on the top side of the pcb, between the casing of the quartz and the board itself. After de- and resoldering it, all OSSCs worked.