Relapse rhymes with:
claps, collapse, elapse, flaps, lapps, laps, lapse, slaps, apps, cap's, capps, caps, chaps, claps, collapse, crapps, craps, elapse, flaps, gap's, gaps, japs, kappes, kaps, knapp's, lapps, laps, lapse, maps, naps, perhaps, raps, saps, schnapps, scraps, slaps, snaps, straps, taps, traps, wraps, yaps, zaps
Relapse sounds like:
railbike, ralph's, ralphs, relapses, reliefs, relieves, roelofs, rohlfs, rolfes, rolfs, rollback, rollbacks, rulebook