Extract audio data from a bagfile into mp3 or wav format

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Hello ROS Community,

I am a beginner to ROS. I used the audio_capture package provided by ROS Wiki to capture the audio data from a usb-microphone and use rosbag record to record the audio topic into a bagfile. Using the audio_play package provided, I was able to hear the audio when I rosbag play the bagfile. Now I want to extract the audio data of the bagfile to a external mp3 or wav file, but I don't know how to.

I know that the audio_capture package does contain a capture_to_file.launch file to capture real-time audio into mp3 file, but I only want the historical audio data recorded by the bagfile.

Regards.