If your system is skipping the .flac audio is likely having buffering issues - often caused by Network/WiFi interference .. perhaps see if the following fixes the matter:

If running your Sonos system on your local WiFi then try setting the routers 2.4ghz band to a ‘fixed’ non-overlapping channel, either 1, 6 or 11. Try each one to see which perhaps proves to be the best signal for your environment (give them 30-40 minutes each, before testing another channel). If problems persist consider using the in-built SonosNet connection between Sonos devices, that’s assuming you have more than one. You can read about that in these two links:

Choosing between a wireless (WiFi) and wired (SonosNet) Setup

Switching a Sonos Household between wireless & wired mode & vice versa

Thanks. I will try this.

Check you don’t have Crossfade enabled, that will deliberately truncate all playback.

Thanks. Not enabled but will explore it.

Crossfade will switch itself off anyway a few seconds after enabling whilst playing a Sonos Radio channel, or so I believe, but always worth a check.