I have encountered a similar problem when connected to a LG TV via HDMI
It appears to happen if you start the 'box' with the TV in standby mode. However, it does not always cause a problem, so I suspect that it is a timing issue in the negotiation phase of the HDMI connection (if you turn the TV on within a few seconds of starting the 'box' it seems to work ok).
When the sound does fail it only seems to be necessary to power cycle the 'box' (turn it off (for about 10 seconds) and on at the mains), leaving the TV switched on