Knapp's rhymes with:
naps, schnapps, snaps, apps, cap's, capps, caps, chaps, claps, collapse, crapps, craps, elapse, flaps, gap's, gaps, japs, kappes, kaps, lapps, laps, lapse, maps, naps, perhaps, raps, relapse, saps, schnapps, scraps, slaps, snaps, straps, taps, traps, wraps, yaps, zaps
Knapp's sounds like:
kamp's, kamphaus, kamps, kemp's, kempke, kempski, kenefick, kennebeck, kimwipes, knapik, knapke, knapsack, knaves, knbc, kneafsey, knives, knobs, konopka, kosanovich, kosnovsky, kosnovsky's, koznovsky, koznovsky's