That sounds like Sonos limitation. With S2, Sonos speakers now have support for high resolution audio, 24-bit, 44.1/48kHz for FLAC/ALAC only.
I don't think I can appreciate the difference on any of my Sonos speakers. They are not audiophile critical listening level speakers anyway but are fantastic for background/casual listening. Sonos room tuning goes a long way towards a well balanced, good sounding but not-audiophile critical grade speaker.