Harder rhymes with:
ardor, carder, gaarder, larder, marder, yarder, ardor, boarder, border, camcorder, carder, corder, crossborder, dedeurwaerder, disorder, gaarder, gorder, herder, larder, marder, norder, order, recorder, reorder, snowboarder, vanorder, warder, weirder, yarder
Harder sounds like:
hardier, hardware, harter, hartter, heartier, herder, hereditary, herter, horter, howerter, huerter