Last night I sang in a concert with the Ebor singers. The first half was mostly Benjamin Britten's well-loved Ceremony of Carols, and the second half was assorted seasonal pieces, interspersed with the Advent ("Big O") Antiphons (O sapientia et al.). It was all quite effective, and very enjoyable. My biggest contribution was working out the approximate Middle English pronunciations of the poems used in the Ceremony of Carols. Here is what I came up with. (The document is in three-column format, with the texts on the left, the glosses in the middle, and a phonetic pronunciation on the right.) Others are welcome to use it; please just drop me an e-mail to let me know if it was useful to you. I drew on my (sketchy) prior experience with Middle English, and also a Google search, which yielded several webpages of which the most concise and helpful was at pronunciationguide.org .
Updated April 3 2012 with a link to a google document version of the text. Use at will, and let me know how it works out for you.