Since the compression algorithm is well know, they can just adjust bit strings to avoid being changed by compression routines and stay intact in the output file. But i dont thing they are doing that on commercial cds, just because it wont identify anything.
[This is something I've been curious about. If you convert some music to MP3 format,
can metadata from the original CD survive the conversion? And what kind of signatures
does your software add into the file? Fire up the old hex editor and start looking
for suspicious stuff.