The present invention provides an audio processing apparatus for automatically controlling gain. The audio processing apparatus includes an audio signal source and a sound reproduction device. The sound reproduction device attaches to the audio signal source, which includes a storage unit for storing a default gain value and a gain index table. The gain index table lists genre types with a genre gain value of each of the genre types. After receiving a play command signal, the audio signal source fetches an audio file to be played, reads a genre type of the audio file from a tag thereof, and processing the audio file to generate audio signals. The audio signal source further reads a genre gain value of the genre type from the gain index table, amplifies the audio signals by the genre gain value, and sends the amplified audio signals to the sound reproduction device to reproduce corresponding sounds.