I am experiencing frequent (inevitable) loss of voiceovers and sound effects in cutscenes.

I will have speech initially (eg main quest lines) but as soon as the game loads the next set of data, all main sound effects are lost - speech and effects. I still retain ambient sounds.
Alt-tab switching also appears to cause the effect.

Other system sounds remain intact throughout.
I have my system set to headphones (2 speaker sound).

Googling the problem I notice this appears not to be an isolated problem but I cannot find a solution.

Thanks in advance for any help!