Relax rhymes with:
black's, blacks, blacks', flax, lac's, lacks, lax, plack's, plaques, plax, slacks
Relax sounds like:
rail's, rails, railway's, railways, raleigh, raleigh's, rales, rales', rales's, raleses, rallies, rallis, ralls, rally's, rauls, rawles, rawls, realize, realizes, reels, reilly's, relaxes, relays, release, releases, relic, relics, relies, relig, religious, relish, relishes, rheology, rials, riles, riley's, rill's, riyals, roelke, rohlicek, roles, rolex, roll's, rolls, rolls's, roulac, roulhac, rowles, royal's, royals, royals', ruelas, rule's, rules, ryals, ryles