Ok, figured this one out so thought I'd post the answer just in case anyone else runs into this problem.
Someone on another forum suggested changing the audio settings from digital to analogue as this worked for them. Went back to the xbox to try this but to my surprise, the file played with sound as soon as I tried it (i/e without changing the audio settings).
Watched something else later on in the day and thought I'd try the original file to see what the sound was doing, it had disappeared again. Tried the suggestion of changing to analogue and lo and behold, sound started coming from the TV. Seemed to be consistant this time - change to analogue = sound, change to digital = sound.
I've got my xbox hooked up to my amp using an optical equipped AV cable, turned the amp on and guess what - there's the sound in digital mode.
Very strange - not sure if this is a bug or not as I would assume that even the digital audio stream this file has would be output to the TV (through the usual connectors) just like a DVD does, as well as the optical channel.
Anyway, now you know (IMG:
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