Not sure what's going on with your DAW then, perhaps installing the LAME MP3 encoder might help. Can your DAW encode to 24-bit/48khz FLAC at all? If you save the FLAC direct to WAV, it will probably still be 96khz, hence why the Sonos will still have trouble with it. You DAW should be able to downsample to 48khz though, then I would still recommend FLAC over WAV has it saves a lot of space with no affect on quality. For most, using a DAW for this would be overkill, but if you already have one you may as well try and get it working!
I have a copy of dBPowerAmp that I use for ripping CDs and converting to FLAC, but it's not free. MusicBee is my favourite music player app and that supports ripping and conversion, perhaps give that a shot?