My suggestion was not related to cables causing the delay, but more so that the codecs on the transmit and receive end for the coax and optical are different silicon/hardware within the units, typically shared silicon on the same IC, but also a different functional block within the audio codecs if they are hardware implementations in silicon. Hence, they could present the issue for the synch with the digital video stream depending on their implementations in hardware. There could also be an implementation where they are software codecs that runs on a DSP which can still present a problem.