Is there any chance you have some program running (or that auto-runs when a CD is inserted) that recognizes audio CDs and starts doing something with them in the background? It is possible, if I read the mmc command specifications correctly, for software to prevent a disc from being unloaded (via the PREVENT ALLOW MEDIUM REMOVAL command). So maybe some background process that only applies to audio CDs is running, recognizing the audio CD, and locking it in the drive as part of its processing.
Check and see what all process are running on your system when you have a audio CD in.
It may also be that some software you are using is locking the CD in but failing to unlock it. Try to just put the CD in and then eject it a minute later without running any programs (including auto-run/auto-play programs).