How do I concatenate two MP4 files using FFmpeg?
To concatenate two MP4 files using FFmpeg, you can follow the steps below:

    Open a command prompt or terminal window.

    Navigate to the directory where your MP4 files are stored using the cd command.

    Run the following FFmpeg command to concatenate the two MP4 files:

```css
ffmpeg -i input1.mp4 -c copy -bsf:v h264_mp4toannexb -f mpegts intermediate1.ts
```

This command will convert the first input file (input1.mp4) into an intermediate transport stream file (intermediate1.ts) in MPEGTS format.

Run the following command to convert the second input file (input2.mp4) into another intermediate transport stream file (intermediate2.ts):

```css
ffmpeg -i input2.mp4 -c copy -bsf:v h264_mp4toannexb -f mpegts intermediate2.ts
```
Run the following command to concatenate the two intermediate transport stream files into a single MP4 file:

```go
ffmpeg -i concat:intermediate1.ts|intermediate2.ts -c copy output.mp4
```
This command will concatenate the two intermediate transport stream files (intermediate1.ts and intermediate2.ts) using the concat protocol and copy the video and audio codecs without re-encoding (-c copy) to create the output file (output.mp4).

After the command is complete, you should have a single MP4 file (output.mp4) that contains the concatenated content of the two input files (input1.mp4 and input2.mp4).

Note: The -bsf:v h264_mp4toannexb option is necessary to ensure compatibility with some video players that require the H.264 video codec to be in the Annex B format. If you encounter any issues with playback after concatenating your MP4 files, you may need to use this option or try another codec.
I want to extract the audio from the output.mp4 file and I want the audio to be in the .opus format and in a 128kbps bitrate. I want to output that into my_sound.opus. What FFMPEG command can do that for me?
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.