Dumbfounded rhymes with:
abounded, appended, astounded, banded, befriended, bended, blinded, branded, broken-winded, commended, comprehended, confounded, contended, corresponded, disbanded, distended, evenhanded, expended, fended, grounded, heavyhanded, hounded, impounded, lefthanded, long-winded, masterminded, mended, overfunded, pounded, rebounded, redhanded, refunded, remanded, reoffended, reprimanded, rescinded, responded, righthanded, sanded, seconded, short-winded, single-handed, transcended, trended, unamended, unattended, unbounded, unbranded, underfunded, underhanded, unfounded, unfunded, unintended, winded
Dumbfounded sounds like:
dampened, downpayment, dumbfound, dunavant, dunnavant