Perhaps the most interesting recordings easily available are those of plainchant from Picardie recorded in a parish church in Vaudricourt, in 1971. This should sound close to plainchant sung in churches during the 19th c., before Solesmes' restotation of "medieval" chant. Here is one example. You could find several more on dailymotion.com.
http://www.dailymotion.com/video/x52ggr_plain-chant-picard-a-vaudricourt-1_music
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