e39 audio mute problem

I have a 2001 e39 Uk spec 530 with the sat-nav/TV option. There is an issue that the audio mutes occasionally. It seems all audio sources are muted and it can usually be recovered by switching
to TV mode. The last owner said it didn't do it before he had a hands- free kit removed, however, given the number of other undisclosed problems I have had: a) what he said can be mostly discounted. b) i have had the area where it would have been fitted inspected by a professional who thought it looked ok.
Any ideas where I could start? I am not sure taking it to a car radio shop would help as continuity checks will pass 99% of the time. Even my local BMW main dealer didn't want to know.
I was wondering if it could be a software issue (if the culprit is a loose wire, I am surprised it can be recovered by switching to TV mode). Is there a way to ascertain what version of software the audio system is running and, if so, is there a way to update it?
Thanks, Andy

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