I had the strange experience as I was studying aural music theory in college, that I began to think of every melody that I heard in solfege. You know, "do, re, mi." Instead of having songs stuck in my head, like most people occasionally get, I would hear a melody--and find myself singing it mentally to solfege! I was only sitting in an aural music theory class twice a week for an hour, and I did struggle in the class, but I passed, so I'm not sure how my mind got so attached to solfege. I would even play little mind games with myself, "wait Mara, what if this melodic phrase actually started on DO instead of SOL; is there really a way to tell for certain?" Like, I see how that can sometimes be applied to chant...is it "so-la-do," or "do-re-fa?" And I hated it! Just like how everyone else hates it when they get a song stuck in their head! It must have been for 2 solid years that I am pretty sure I was constantly hearing solfege. Gosh, it was annoying. At some point, however, it has thankfully dwindled away. I can't give any sort of precise date, but I will say that it now only happens when I think somewhat consciously about it.