How to combine multiple audio files at a specific time into a video file in Android?

Question

How to combine multiple audio files at a specific time into a video file in Android?

What I want to do is merge several audio files with a video file at a specific time in my Android project.

For example, the presence of two audio files: a1 with a length of 5 seconds, a2 with a length of 5 seconds and video V1 with a length of 20 seconds. Therefore, I want to combine audio a1 in video V1 at time 00: 01s to 00: 05s, audio a2 in video V1 at time 00: 10 s 00: 15 s. “Combine” here I meant that the video file will add voice acting from the a1 and a2 audio files at a certain time, and not add it.

After searching on Google and StackOverflow, I found the Mp4Parser library, but unfortunately I can completely merge the audio track into the video track. My question is how to combine it at a specific time and with multiple audio tracks?

Here is the code I used:

    public boolean mux(String videoFile, String audioFile, String outputFile) {
    Movie video;
    try {
        video = new MovieCreator().build(videoFile);
    } catch (RuntimeException e) {
        e.printStackTrace();
        return false;
    } catch (IOException e) {
        e.printStackTrace();
        return false;
    }

    Movie audio;
    try {
        audio = new MovieCreator().build(audioFile);
    } catch (IOException e) {
        e.printStackTrace();
        return false;
    } catch (NullPointerException e) {
        e.printStackTrace();
        return false;
    }
    Track audioTrack = audio.getTracks().get(0);
    List<Track> tracks = new ArrayList<Track>();
    tracks.add(video.getTracks().get(0));
    tracks.add(audioTrack);
    video.setTracks(tracks);
    Container out = new DefaultMp4Builder().build(video);
    FileOutputStream fos;
    try {
        fos = new FileOutputStream(outputFile);
    } catch (FileNotFoundException e) {
        e.printStackTrace();
        return false;
    }
    BufferedWritableFileByteChannel byteBufferByteChannel = new BufferedWritableFileByteChannel(fos);
    try {
        out.writeContainer(byteBufferByteChannel);
        byteBufferByteChannel.close();
        fos.close();
    } catch (IOException e) {
        e.printStackTrace();
        return false;
    }
    return true;
}

private static class BufferedWritableFileByteChannel implements WritableByteChannel {
    private static final int BUFFER_CAPACITY = 1000000;

    private boolean isOpen = true;
    private final OutputStream outputStream;
    private final ByteBuffer byteBuffer;
    private final byte[] rawBuffer = new byte[BUFFER_CAPACITY];

    private BufferedWritableFileByteChannel(OutputStream outputStream) {
        this.outputStream = outputStream;
        this.byteBuffer = ByteBuffer.wrap(rawBuffer);
    }

    @Override
    public int write(ByteBuffer inputBuffer) throws IOException {
        int inputBytes = inputBuffer.remaining();

        if (inputBytes > byteBuffer.remaining()) {
            dumpToFile();
            byteBuffer.clear();

            if (inputBytes > byteBuffer.remaining()) {
                throw new BufferOverflowException();
            }
        }

        byteBuffer.put(inputBuffer);

        return inputBytes;
    }

    @Override
    public boolean isOpen() {
        return isOpen;
    }

    @Override
    public void close() throws IOException {
        dumpToFile();
        isOpen = false;
    }
    private void dumpToFile() {
        try {
            outputStream.write(rawBuffer, 0, byteBuffer.position());
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
}