BackgroundWorker won't run twice

You can do this using a background worker fine, but you're trying to do some of the work in the process button event handler and only some in the background worker.

Try this...

* Drop a BackgroundWorker component on your form. Name it 'audioWorker'
* Click on it and choose properties
* Set WorkerReportsProgress and WorkerSupportsCancellation to true
* Click on events and create event handlers for DoWork, ProgressChanged and RunWorkerCompleted

Add a new class to your project. Call it ProcessAudioStartArguments

/// <summary>
/// A class to pass as an argument to the DoWork method of the background
/// worker
/// </summary>
public class ProcessAudioStartArguments
{
    /// <summary>
    /// ctor
    /// </summary>
    /// <param name="files">List of file names to process</param>>
    public ProcessAudioStartArguments(List<ProcessStartInfo> files)
    {
        this.FilesToProcess = files;
    }
    /// <summary>
    /// The list of files to delete
    /// </summary>
    public List<ProcessStartInfo> FilesToProcess { get; private set; }
}

Your actual code should then look something like this..

* Build up a list of arguments to send to the background worker
* Start the worker, pass in our custom arguments object

private void btnProcess_Click(object sender, EventArgs e)
{
   List<ProcessStartInfo> files = new List<ProcessStartInfo>();

   // Prepare your arguments to send to the worker...
   foreach (string loop in fileNamesToProcess)
   {
      Prepare(loop, out thePath, out tempStr, out thecommand, out thearguments);

      ProcessStartInfo psi = new ProcessStartInfo();
      psi.WindowStyle = ProcessWindowStyle.Minimized;
      psi.Arguments = thearguments;
      psi.FileName = thecommand;

  files.Add(psi);
   }

   ProcessAudioStartArguments arguments = new ProcessAudioStartArguments(files);
   audioWorker.RunWorkerAsync(arguments);
}

* Do all processing in the worker thread. Check for cancellations, report on progress etc

private void audioWorker_DoWork(object sender, DoWorkEventArgs e)
{
   ProcessAudioStartArguments arguments = (ProcessAudioStartArguments)e.Argument;
   foreach(ProcessStartInfo psi in arguments.FilesToProcess)
   {
     if (audioWorker.CancellationPending)
         break;

     // Some sort of progress reporting...
     var displayText = string.Format(Resources.ProcessingFile, Path.GetFileName(psi.FileName));
     audioWorker.ReportProgress(step, displayText);

     Process p = Process.Start(psi);
     p.WaitForExit();

     // Some sort of check for success? Return code of process?

     // Delete the original
     File.Delete(workFile);
     //  Rename the processed file
     File.Move(outFile, workFile);
   }
   e.Result = someObj // Report on the success of the processing...?
}

Handle progress reporting...

void audioWorker_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
   string currentFile = e.UserState.ToString();

   // Some sort of process window to display percentage done etc...
   progressWindow.SetText(currentFile);
   progressWindow.StepTo(e.ProgressPercentage);
}

So leave all the work to the background worker, you just start it once and pass in an arguments object that gives it all the information it requries to complete the job.

I do something similar in my article Secure Delete .NET[^].

HTH

There are several issus that I can see with your code.

The first, most obvious one, is that you are allocating a new event handler for DoWork in the loop every time, but you don't deallocate it anywhere.

The second thing is that while you are looking to try and do something in a background thread, the while loop inside your foreach means that you've created a tight loop in there which chokes the primary thread. As a result of this, you've blocked the main thread.