Play method cannot find concatenation .wav file (delay in creation)

Question

This util method helps with creating, silencing and concatenating selected .wav files using bash commands within unix. For some reason when I'm running the methods given a list of .wav files in a list, the concatenation of .wav files only appears when I run the same method twice. Thus the playAudio() method gives me an error of:

java.io.FileNotFoundException: temp/combine.wav (No such file or directory)
at java.io.FileInputStream.open0(Native Method)
at java.io.FileInputStream.open(FileInputStream.java:195)
at java.io.FileInputStream.<init>(FileInputStream.java:138)
at java.io.FileInputStream.<init>(FileInputStream.java:93)
at namesayer.util.PlayAudio$1.call(PlayAudio.java:35)
at namesayer.util.PlayAudio$1.call(PlayAudio.java:30)
at javafx.concurrent.Task$TaskCallable.call(Task.java:1423)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.lang.Thread.run(Thread.java:748)
java.io.FileNotFoundException: temp/combine.wav (No such file or 
directory)
at java.io.FileInputStream.open0(Native Method)
at java.io.FileInputStream.open(FileInputStream.java:195)
at java.io.FileInputStream.<init>(FileInputStream.java:138)
at java.io.FileInputStream.<init>(FileInputStream.java:93)
at namesayer.util.PlayAudio$1.call(PlayAudio.java:35)
at namesayer.util.PlayAudio$1.call(PlayAudio.java:30)
at javafx.concurrent.Task$TaskCallable.call(Task.java:1423)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.lang.Thread.run(Thread.java:748)

My class is:

//Input of a string with destination of audio file to play - uses AudioStream and AudioPlayer for .wav files
public class PlayAudio {
private String audio;
private List<String> combineAudio = new ArrayList<>();
private AudioStream audioStream;

public PlayAudio(String audio) {
    this.audio = audio;
}

public PlayAudio(List<String> combineAudio) {
    this.combineAudio = combineAudio;
    audio = "temp/combine.wav";
}

public void playAudio() {
    Task task = new Task<Void>() {
        @Override
        protected Void call() throws Exception {
            InputStream in = null;
            try {
                in = new FileInputStream(audio);
                audioStream = new AudioStream(in);
                AudioPlayer.player.start(audioStream); // play the audio clip with the audio player class
            } catch (IOException e) {
                e.printStackTrace();
            }
            return null;
        }
    };
    new Thread(task).start();
}

public void playCombinedAudio() {
    //Silence noise sections of each audio
    for (String audio : combineAudio) {
        String silenceAudio = "cd data/names\n" +
                "cp " + audio + ".wav ../../temp/" + audio + ".wav\n" +
                "cd ../../temp\n" +
                "ffmpeg -hide_banner -i " + audio + ".wav -af silenceremove=1:0:-30dB:1:5:-30dB " + audio + "CONCAT.wav\n";
        ProcessBuilder processBuilder = new ProcessBuilder("bash", "-c", silenceAudio);
        try {
            processBuilder.start();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    //Combine audio clips together
    FileWriter writer = null;
    try {
        writer = new FileWriter("temp/combineAudio.txt");
        for (String str : combineAudio) {
            writer.write("file '" + str + "CONCAT.wav'\n");
        }
        writer.close();
    } catch (IOException e) {
        e.printStackTrace();
    }

    //Concatenate from txt file
    String concatAudio = "cd temp/\n" +
            "ffmpeg -f concat -safe 0 -i combineAudio.txt -c copy combine.wav\n";
    ProcessBuilder processBuilder = new ProcessBuilder("bash", "-c", concatAudio);
    try {
        processBuilder.start();
    } catch (IOException e) {
        e.printStackTrace();
    }

    playAudio();
}