Zero sound experiments on superfluid ³He-A at low pressures (< 3 bar) and in strong magnetic fields (> 0.3 T) show an increase in the attenuation at low temperatures (≤ 0.4 mK) which has been a puzzle until the present experiment. Coupling of the re-entrant normal flapping (NFL) mode to zero sound in a geometry where sound propagates in the direction of the magnetic field is shown to arise from a boundary layer at the walls whose thickness is about the dipolar coherence length ξ_D of ³He-A. We estimate the value of ξ_D at low temperatures and at low and intermediate pressures. We have also studied the nonlinearity of sound propagation near the NFL mode and found a reversal of it when passing the mode.