The Toppy does not seem to cope very well with playing audio if the TV type is set different to the video being played. In the UK, all broadcasts are PAL so the TV Type should be set to PAL, and this can be changed using the System Setting menu, A/V Output Setting item.
There is also a known issue with (specifically) Film4 recordings being played back when tuned to Sky3 (or any channel on MuxC). The solution is to tune to a different channel (e.g. BBC1) before starting playback.
Other causes are the obvious signal problems: weak signal and poor quality signal (check the aerial type, alignment and cabling, water ingress in connections, and consider a booster), atmospherics, and interference from other electrical activity, such as lawnmowers, car electrics and dodgy domestic appliances (better [CT100] cabling may help).
Experience has shown that audio dropouts can occur when the machine has got hot, often after a few hours playback. This gets progressively worse. The solution is either switch off and allow it to cool down, or direct a cool air fan at the machine. The fan noise can be ignored by the satisfaction of completing a watched programme.