It would have been nice if LP modes were available in Red Book CDs (no compression but lower bit depth and/or mono). The motor controller can already handle a wide range of speeds to maintain a constant linear velocity. And if that turned out to be technically difficult, automatic two-pass mono could work - include an extra bit in the ToC for L/R channel, and play one of them first and then the other with the second half of the tracks. The result would be a double-runtime mono disc with a seek break in between two tracks around the middle (varying in length between players) but that would be acceptable for audiobooks. Of course CD-ROMs with MP3s/AACs are available but player compatibility is still problematic.