Weird sound probs on (Granada) PLUS

Whilst flicking through the channels on Sky Digital just now, I had to stop at PLUS, where
the old episode of "London's Burning" being shown, is being played with only background
sounds, but absolutely no vocal sounds. It's as though all the characters are just lip-syncing!


EDIT: After coming out of an ad-break, it's appears to have returned to normal.
Any ideas as to what could've been causing the aforementioned sound glitch?

Most broadcast tape formats (DigiBeta, D3 etc) have four audio tracks. Although all broadcasters have their own technical specification for transmission tapes, it's not uncommon to find a full mix of English dialogue on Tracks 1 and 2 (Stereo Left and Right) and what's known as Music and Effects (M&E) on Tracks 3 & 4. The reason for this is so that foreign broadcasters can dub their own local language and mix in clean Music and Effects to make their own finished programe.

Sounds like Plus were inadvertently transmitting only the M&E tracks for Part1 of the prog !