ut queant laxis
That was a bit surreal. Radio 3 just played a very nice setting of Ut queant laxis and didn't so much as mention Guido d'Arezzo or solfeggio.
(Apparently some cathedral somewhere has, or had, a musical clock that plays the hymn; Iain Burnside is celebrating The Day The Clocks Go Back with a set of time-related pieces. I don't think the version of the hymn they played has the same melody as the plainchant Guido used.)