An AI analysis, not guaranteed 100% accurate...
"The most likely problem is that the FLAC files aren’t encoded in a Sonos-friendly way, even though they’re technically valid FLAC. Sonos supports local-library FLAC only up to 48 kHz and mono/stereo files, and some Sonos documentation also notes FLAC/ALAC bit-depth limits depending on S1 vs S2; files outside those limits or with odd encoding metadata can fail even when other players handle them fine.
Why Audacity “fixes” them
When you open a file in Audacity and re-export it, Audacity rewrites the audio stream and container metadata cleanly. That often strips or normalizes things Sonos is picky about, such as unusual sample-rate flags, channel layouts, or metadata structures, which explains why the re-exported version plays properly.
What’s probably happening
Common causes are:
- The source FLACs are actually 96 kHz or 192 kHz, which Sonos rejects even if the bitrate looks fine.
- The files have more than 2 channels or some nonstandard channel mapping, which Sonos local-library playback does not support.
- The FLAC encoding/tagging produced by your converter is legal but not robust enough for Sonos, and Audacity’s export creates a cleaner file structure.
Practical fix
Re-encode the library to Sonos-safe settings: stereo, 44.1 kHz or 48 kHz, and standard FLAC export from a trusted encoder. Audacity’s export path works because it rewrites the files with conventional settings, and Sonos community guidance specifically recommends 48 kHz FLAC for compatibility.
Fast way to confirm
Check one failing file’s actual properties with a media inspector: sample rate, channels, bit depth, and whether it contains anything beyond standard stereo FLAC. If the failing files are 96 kHz/192 kHz or otherwise nonstandard, that’s the explanation, not the conversion brand itself.
The short version: Sonos is usually not objecting to “FLAC” in general; it’s objecting to the exact encoding details of those particular FLAC files."