iTunes uses the file extension .m4a for AAC
unprotected tracks and also for Apple Lossless tracks (and these are unprotected as well).
In the past the iTunes Store sold DRM protected AAC tracks using Apple's "FairPlay" DRM scheme and these used the file extension .m4p. Sometime ago all the record companies (partially) came to their senses and the iTunes Store now sells unprotected music files.
Never has iTunes ripped from CD to .m4p, it has always ripped to .m4a.
Note: .mp4 and m4p are not the same thing, .mp4 is a video file, .m4p is a protected AAC audio file.
Look in iTunes itself and see what it thinks the tracks are, if you either do a get info on the track(s) or view in column view and turn on the 'kind' column it will list them as either
Protected AAC audio file, or
AAC audio file, or
Apple Lossless audio file.
If they are listed as unprotected in iTunes then it maybe that the Sonos is being confused. As these are older files the format might have changed slightly in different iTunes versions. If they are unprotected you could try telling iTunes to convert them from AAC to AAC or Apple Lossless to Apple Lossless and by using the latest iTunes this may help.
I don't have a Sonos, but do have a Roku SoundBridge (similar) and it has no problems with my Apple Lossless files, and neither does Windows Media Player or Microsoft Media Center. (Yes WMP can be made to play Apple Lossless.)
Note: All my tracks have been ripped from CDs and not downloaded.
