As you are able to access the CD source, is it showing that there is currently a disc in the system?
If you were to switch from the CD/DVD source to another source on the console, then back to the CD/DVD, are you able to open the disc tray then?
One thing I would definitely recommend trying is a reset of your control console. To do this what you will want to do is:
Power off your system
Unplug the power cord from the Acoustimass bass module for 30 seconds
Wait for Bose 321 to appear and then disappear from the system console display
Once Bose 321 has disappeared from the display, try opening the disc tray before you power on the system fully.
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