Undoing rhymes with:
dewing, doing, outdoing, overdoing, redoing, subduing, accruing, blueing, bluing, booing, brewing, canoeing, chewing, cooing, cuing, debuting, dewing, doing, eschewing, ewing, hewing, lewing, misconstruing, nonaccruing, outdoing, overdoing, pursuing, queuing, redoing, renewing, reviewing, screwing, skewing, spewing, stewing, subduing, suing, tattooing, viewing, wooing
Undoing sounds like:
undying, unending, uniden's, uniting, unwinding, unwitting