Can anyone offer any experience as to why the sound would sometimes work and sometimes be silent? Once silent, it would seem to come back on again after a while or once restarted - with no particular pattern.

It has the Harman Kardon speakers and the stock radio/cassette head unit.

Any suggestions very gratefully received!

 

Did you ever figure out what the problem was? I'm having the same problem with my Harman Kardon system in my '06 LR3 HSE.

 

I would have to say it is RFI (radio frequency interference)
Either from the alternator or a bad ground or bad plug wires if you have the petrol engine.