As far as I can see (and I'm definitely no expert) this happens - to me anyway - only with downloaded content. It usually starts with the ad break being mute, then sometimes when it gets back to the show the sound is back on, sometimes not. I've found that if you wait and come back to it it will come on. I feel it may be due to slower than usual download of the programme. Video is downloaded but if you watch it as soon as it says "available", it's like the audio has to catch up. So when you get to the point where, to continue the theory, the audio has not yet downloaded, it's not there so you can't hear it. Almost every time it's happened to me I have done nothing but pause it and go back maybe 10 mins or so later and it works fine. Just now it worked when I simply ran the progamme back a few seconds. Don't know if it always will.
But that's my guess: it's to do with somehow the audio not being downloaded to your box at the same speed as the video. Makes sense when you think about it. I had a similar problem with a programme being "available" but if I scooted through ads it suddenly said "end of programme." The video and audio at that stage, ie the whole file, had not yet downloaded fully and so I had to wait for it to do so. FYI i have fast broadband (360 Mbps with Virgin) so it's not my broadband connection being slow.
Dunno if that helps, but it is another thing to try. The other option (boring as it is) is to watch your download till the percentage is above 50% and then start watching it. Usually by the time you get to the end of the programme the download has caught up.
