The invention provides a sound-reproducing apparatus and a high frequency interpolation-processing method in each of which high frequency interpolation processing having a light burden imposed thereon can be executed. Compressed audio data is decoded by a decoder (202a) and the resulting audio signal is transmitted to a harmonizer (209a). Auxiliary information such as a sampling rate and a bit rate contained in the decoded audio signal is read out by a header reader (209b) and is transmitted to a harmonizer (209a). Communication of signals from/to respective portions is made through a bus (210). The harmonizer (209a) has a preset for an interpolation band obtained in accordance with a format and a sampling frequency of the compressed audio data, and executes processing by referring to file information transmitted thereto from the header reader (209b).