Here is the (almost) complete list of folk songs.
http://anntheword.blogspot.com/2011/07/folk-songs-from-pasolinis-canterbury.html

The specific recording of Hal an Tow used is here about 47 seconds in.

https://www.youtube.com/watch?v=zmmP9wibT1s

One folk song is still missing though. The (Gaelic?) song sung by a woman when Alan and John are searching for their missing horse in the Reeve's Tale.