For the COAX input, I have to say we made a mistake for this board, it did not follow the SPDIF specification for coaxial which has input voltage 0.4Vpp~0.6Vpp. We just connected the connecter into MCU directly, and it will expect 0~3.3V, which is not suitable to connect to other digital player… for now, please use optical only.
Also, the COAX and output of Optical adapter are sharing the sampe SPDIF input to MCU, so they will interference with each other.