The response is derived from web search results. FFMPEG is a command-line tool that can be used to convert audio and video files between different formats. To extract the audio from the output.mp4 file and convert it to the .opus format with a 128kbps bitrate, you can use the following command:

```
ffmpeg -i output.mp4 -c:a libopus -b:a 128k my_sound.opus
```

This command will take the output.mp4 file as an input, use the libopus codec to encode the audio stream, set the audio bitrate to 128k, and output the result to my_sound.opus¹²³. You can also add other options to adjust the quality or metadata of the output file⁴⁵.

Source:
1. Lossless extraction of audio from .webm and .mp4 files. https://superuser.com/questions/1750645/lossless-extraction-of-audio-from-webm-and-mp4-files
2. How do I extract Opus frames from audio file with FFmpeg?. https://superuser.com/questions/1764480/how-do-i-extract-opus-frames-from-audio-file-with-ffmpeg
3. audio - Extracting Opus from WebM losslessly with FFmpeg or MKVToolNix .... https://superuser.com/questions/1474050/extracting-opus-from-webm-losslessly-with-ffmpeg-or-mkvtoolnix
4. Lossless extraction of audio from .webm and .mp4 files. https://superuser.com/questions/1750645/lossless-extraction-of-audio-from-webm-and-mp4-files
5. Convertible: using ffmpeg to convert audio files – quantixed. https://quantixed.org/2021/11/20/convertible-using-ffmpeg-to-convert-audio-files/
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