For phonons propagating through superlattices, "intermode" Bragg reflections due to the coupling of different polarization modes can occur besides ordinary "intramode" Bragg reflections, and corresponding frequency gaps are classified according to the types of Bragg reflection. We demonstrate the difference between localized modes generated in different types of frequency gaps in superlattices with inhomogeneity. In particular, we show for a superlattice with a free surface, a "perfect" resonant reflection associated with surface-localized modes is realized inside frequency gaps in which intermode Bragg reflections occur. This resonance can be interpreted as resonant tunneling through a symmetric double-barrier potential.