Using my TalkTalk box directly to TV (not through any other sound system) the synch of sound drops out. OK, it is reset if I switch the box off and on. However, this is a real bind especially if on catch up because the programme then, sometimes, goes right back to the beginning or has to be reloaded.
The sound and picture device synchronisation can be pre-set on the TV or on the TV Box but once set it should remain in place.
On the TV Box have a look at Settings > Picture & Sound and see what the sound synchronisation setting is. It might say -1ms which is the odd default for the YouView software or it might be 0ms, assuming you've not previously pre-set a fixed figure up to around the 200ms time period.
The soft reset of the TV Box often works to fix strange operating faults. Press and hold the YouView front panel power / standby button on for 8 seconds, until the lights change then let go. Then just wait for the box to restart.
If that doesn't fix it we can move on to trying a maintenance reset.