[mgpai]

EDIT by pspzockerscene:
If you were sent to this thread but you do not know what the EventScripter is, please read THIS support article first.

Convert AAC/M4A/OGG/OPUS files to MP3.

// Convert aac/m4a/ogg/opus files to mp3.
// Trigger required: "A Download Stopped".
// Requires ffmpeg/ffprobe. Uses JD ffmpeg/ffprobe settings if available.
// Overwrites destination file (mp3) if it already exists.

if (link.isFinished()) {
    var fileName = link.name.replace(/(.+)(\..+$)/, "$1");
    var fileType = link.name.replace(/(.+)(\..+$)/, "$2");
    var sourceFile = link.getDownloadPath();
    var audioFile = /\.(aac|m4a|ogg|opus)$/.test(sourceFile);

    if (audioFile) {
        var downloadFolder = package.getDownloadFolder();
        var destFile = downloadFolder + "/" + fileName + ".mp3";
        var ffmpeg = callAPI("config", "get", "org.jdownloader.controlling.ffmpeg.FFmpegSetup", null, "binarypath");
        var ffprobe = callAPI("config", "get", "org.jdownloader.controlling.ffmpeg.FFmpegSetup", null, "binarypathprobe");
        var data = JSON.parse(callSync(ffprobe, "-v", "quiet", "-print_format", "json", "-show_streams", "-show_format", sourceFile));
        var streamsBitrate = data.streams[0].bit_rate ? data.streams[0].bit_rate : 0;
        var formatBitrate = data.format.bit_rate ? data.format.bit_rate : 0;
        var bitrate = Math.max(streamsBitrate, formatBitrate) / 1000;
        var deleteSourceFile = false; // Set this to true to delete source file after conversion.

        if (bitrate > 0) {
            callSync(ffmpeg, "-y", "-i", sourceFile, "-b:a", bitrate + "k", destFile);
            if (deleteSourceFile && getPath(destFile).exists()) deleteFile(sourceFile, false);
        }
    }
}

Alternate version (2021.11.04)

/*
    Convert audio files to mp3 format
    Trigger : A download stopped
*/

if (link.finished) {
    var input = link.downloadPath;
    var output = input.replace(/(aac|m4a|ogg|opus)$/, "mp3");

    if (input != output) {
        try {
            var ffmpeg = callAPI("config", "get", "org.jdownloader.controlling.ffmpeg.FFmpegSetup", null, "binarypath");
            var bitrate = callSync(ffmpeg, "-i", input).match(/bitrate: (\d+) kb/)[1];

            callAsync(function(error) {
                !error && getPath(input).delete();
            }, ffmpeg, "-y", "-i", input, "-b:a", bitrate + "k", output);
        } catch (e) {};
    }
}