I don't now about Sonos but if it were SB3/SB+/Transporter then I'd be checking that the volume control is set to max and not damaging the signal by any volume adjustments.

I'd also check the file conversions table to make sure that it wasn't doing something daft with your input file.

No idea if there are any parallels to Sonos here.

Do the CD's have any form of copy protection? This can impact ripping quality although I have seen a protected disc for 2-3 years now.

Rgds,
Nick