The sound problem in plane geometry is solved under the assumption of no intermolecular collisions and for arbitrary gas-surface scattering kernels, first disregarding any interference from the detector and then with full account of the interreflections. In the latter case the phase lag is found to behave like s log s for small widths of the gap between transmitter and detector. Expansions derived for idealized cases show that measurements of transmitted sound hold little promise of yielding significant information about the gas-surface interaction. Such a goal is better approached by studying the reaction to the transmitter (e.g., by observing the damping of a freely vibrating elastic reed), whereby the normal-momentum accommodation coefficient could be determined.