That's incredible, it's the second time it happens today...

I solved the problem by myself... I hope this will be useful for somebody else, at least...

I have just learnt that I cannot actually mount a cdrom, and I don't need to.

If it's an audio CD it will just start with a multimedia software (kaffeine, xmms..), whereas, you can access to the content of the CD when it's data, either by command line (cd /media/cdrom) or by clicking on the icon on the desktop.

Problem solved.