Plainsong rhymes with:
birdsong, hissong, hussong, hysong, birdsong, budlong, chamlong, daylong, furlong, headlong, headstrong, hissong, hourlong, hussong, hysong, monthlong, scuppernong, ssangyong, weeklong
Plainsong sounds like:
planking, plinking, plunging, plunking, pulmazyme's