Maybe it's quodlibet? Overlaying multiple songs over one another just so precisely so that they don't clash.
For example, Quodlibet from Bach's Goldberg Variations.
https://www.youtube.com/watch?v=mWCwAPf1C9A
Glenn Gould's 1955 Star Spangled Banner/God Save The Queen Mashup
https://www.youtube.com/watch?v=33mk-hgA1mI
Milhaud's 14th and 15th string quartet are stand alone pieces but are also sometimes played at the same time to make an Octet. The Octet version sounds terrible in my opinion though. It seems improbable that bard songs each sound okay by themselves and in combination with each of the other bard songs or in combination with 3 or 4 or 5 or however many you can twist. Maybe the bard transposes certain songs into other keys or shifts them up a 3rd or whatever interval is needed so the songs don't clash.
Playlist with the 14th and 15th
https://www.youtube.com/watch?v=CRd1...9u5m-I&index=4
14th + 15th played at the same time:
https://www.youtube.com/watch?v=2b89IdsdEBA