I would try a few things (assuming Beam is connected over HDMI), firstly on the TV:

1. Toggle off/on both ‘Simplink’ and ‘auto-power sync’ and leave both set to ON.
2. If you have other devices connected to the TV HDMI ports then switch off HDMI-CEC control on those ‘other’ devices

In the Sonos App;

1. Goto "Settings/System/tBeam Room]” and toggle OFF/ON "TV Autoplay’ and then leave it ON

… and see if those few things resolve your ‘loss of sound’ issue.

I’d also be tempted to ‘reboot’ the TV, by unplugging it from the wall for five minutes, ipthen plugging it back in, in none of the excellent recommendations above don’t work. 

No luck. Unplugged both TV and beam and left for a couple of minutes. Toggling simplink off causes it to revert to playing through the TV speakers, then switching it on and changing to External has the same effect as just switching sound source.

Had two other devices: chrome and raspberry pi with Lori connected. Reconnecting the pi causes the silence.

There is likely some CEC-Client software that scans for devices that’s causing the issue on your raspberry pi. You perhaps need to either switch that off, or perhaps install a cec-less adapter and hopefully that ‘may’ resolve your issue.

Ok thanks for the suggestion about other devices triggering it. Turns out a power glitch at home had caused the pi to fault somehow and the locked up kodi was causing it to fail.

I don’t think there is anything that Sonos could do in an update that could have caused this, and i think we’d have seen more posts if it were generic.  Whether that is true or not, some (maybe all) LG TVs seem to have the unfortunate property that turning off HDMI-CEC (Simplink) disables HDMI-ARC.  And HDMI-CEC cannot be applied device-by-device, only globally.

Possible solutions (or experiments):

1. Use the optical out of TV with the Sonos-supplied adapter to connect to Beam
2. Buy a CEC-less adapter for the Raspberry Pi connection to stop it grabbing the TVs attention

Edit: crossed with your post.