On "Sumer Is Icumen In," see this thread: Lyr Req: 'Summer is a comm?in in' ?. The manuscript is HERE. The "Judas" manuscript is HERE (Click on "Folio 34r" to see the enlarged image). One of the earliest known musical manuscripts of European songs is of the late 9th century French liturgy, about which this site (MS 096: ANTIPHONAL: OFFICE OF ST. REMIGIUS (1 OCTOBER)) says, "A rare survival from a very early French musical liturgy, probably among the earliest extant." The "earliest known songs of what might be broadly termed Western culture are the Biblical Psalms." (from HERE) See also this thread: Tune Req: Original Psalm Tunes.

~Masato