In the 29 April Digest Jon Guy was citing his local newspaper, that
wrote: "The oldest known harmonized music performed today is the
English song, 'Sumer is Icumen In', which dates from about 1240."

I doubt it.  This piece of music is written by John Forsite, and is a
canon for six voices.  In fact it is the oldest canon for six voices we
know of.  It is written in the year 1240.  So the newspaper better
had written: "the oldest canon for six voices performed today".

In fact, starting about 1050 the first polyphonic (harmonized) music is
showing up, competing against the old Gregoric monophonic tradition.
And some pieces of this early music are left.  Specialized choirs are
singing this music.

Jan Kijlstra