1. An acoustical-signal processing apparatus, comprising: a feature extracting unit that receives a multichannel acoustical signal and extracts feature data common to a left channel signal and a right channel signal included in the multichannel acoustical signal, based on a composite similarity obtained by combining similarities among the left channel signal and the right channel signal; and a time-base companding unit that receives the multichannel acoustical signal and executes time compression and time expansion of the multichannel acoustical signal based on the extracted feature data.