Have you tried testing with other equipment such as normal terrestrial TV via an aerial , or a games console, Firestick etc?
Also using a different cable and/or port and rebooting your equipment
I had an issue with the loss of my V6 sound last year. I fixed it by having a spring clean. It was like spaghetti junction behind my TV. I removed all leads and cables from all equipment, unplugged them all from the mains. Then reconnected everything and voila the sound was restored.
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I'm just a normal customer here to share my knowledge, I don't work for Virgin Media.
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I have tried cleaning, moving the box, changing the leads, restarting, changing audio settings.
Channels through the aerial are fine, firestick works fine. I only think it is something to do with the box because all of a sudden whilst watching it the sound just went. I checked about faults in the area and they are none.