You can use the following FFMPEG command to extract audio from output.mp4 and convert it into .opus format with a bitrate of 128kbps:

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

Here's what each option does:
- `-i output.mp4`: specifies the input file.
- `-vn`: disables video recording, so only audio is extracted.
- `-c:a libopus`: sets the codec for audio to Opus.
- `-b:a 128k`: sets the bitrate for audio to 128 kbps.
- `my_sound.opus`: specifies the name of your output file.
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