As i wrote here:
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.
Maybe(?… with a big question mark, since you bought a preassembled OSSC) that’s the problem in your case too. It’s an issue you probably can’t see (since the solder bridge could be under the quartz) and it is easy to verify (simply use a multimeter in continuity mode to check, if both pins of the quartz crystal are electrically connected).