Did you set up your sonos sound system in the 360 settings?
If so go to Settings > Audio & Video > Volume keys control and check it's set to your sonos and not the TV.( I've had to change that setting a few times whilst trying to help other users and forgetting to change it back, but it's also easily changed in error)
You could also try and pair the Sonos again as sometimes the first setting it accepts isn't always the best so it's a good idea to try a few more.
Settings > Audio & Video > Pair remote with devices > Change Audio setup
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Thanks for the reply. Im not someone that easily sets things up - technology isnt my thing !!. All ive done is replaced the virgin box and plugged in the new one. Sounds like i need to set up the sonos somehow ?. Ive tried settings but dont see Sonos anywhere. Is there a settings button on the 360 remote ??