As an update – it's not cables or bluray player … Same audio blip was happening with different cables and different bluray player. So it's either :
Amplifier (likely as Onkyo have known issue with HDMI)
Telly (unlikely)
Anyway – dvd playback is working fine with Xbox360 -> Amp, it's just when blurays only have Dolby D soundtracks (instead of HD) where I now have a problem.