1. An audio playback device, comprising: a manipulation unit which receives a command to adjust an audio playback speed from a user; a decoder which, if the command to adjust the audio playback speed is input, decompresses audio data and converts the audio data into an audio stream; a stream processor which divides the audio stream into a high frequency band and a low frequency band, separately signal-processes the audio stream in the high frequency band and the low frequency band to produce a signal-processed-high-frequency audio stream using a first processing method and a separate signal-processed-low-frequency audio stream using a second processing method other than the first method, and integrates the signal-processed high and low frequency audio streams into an integrated signal-processed audio stream; and a playback unit which converts the integrated signal-processed audio stream output from the stream processor into an analog audio signal, and amplifies and outputs the analog audio signal.