Here is the result of our discussion. This may be useful for others to use, so I post it here as a backup.

The audio file opened in Sound Forge consists out of sounds which are 8 seconds long. After each sound is a silent pause of 8 seconds.

The script will apply a pitch shift effect to each sound, then remove the silence and export the sounds to individual files.

using System;
using System.Windows.Forms;
using SoundForge;

public class EntryPoint {
public void Begin(IScriptableApp app) {

   string[] preset = new string[4]; //names of all Pitch Shift presets
   preset[0] = "[Sys] Fifth down";
   preset[1] = "[Sys] Fifth up";
   preset[2] = "[Sys] Third down";
   preset[3] = "[Sys] Third up";

   long regionLength = app.CurrentFile.SecondsToPosition(8.0); //region length in seconds
   long pauseLength = app.CurrentFile.SecondsToPosition(8.0); //pause length in seconds
   long positionFrom = app.CurrentFile.Length - regionLength - pauseLength; //begin at the end of the file because pitch shifting changes its length

   int i = preset.Length-1; //begin with the last preset

   while(true){

      app.CurrentFile.DoEffect("Pitch Shift", preset[i], new SfAudioSelection(positionFrom, regionLength), EffectOptions.EffectOnly | EffectOptions.WaitForDoneOrCancel);

      if(positionFrom <= 0){ //exit loop after the file was processed from end to the beginning
        break;
      }
      
      positionFrom = positionFrom - regionLength - pauseLength; //calculate the positions for the next loop
      
      if(positionFrom < 0){ //the starting position cannot be negative
         positionFrom = 0;
      }

      i = ((i-1)%(preset.Length)+(preset.Length))%(preset.Length); //make sure i stays always inside the array boundries for the next loop
   }
   
   //remove all silence with the Auto Trim/Crop function and export the resulting regions
   app.CurrentFile.DoEffect("Process.AutoTrimCrop", "[Sys] Phrase concatenator 1", new SfAudioSelection(0, -1), EffectOptions.EffectOnly | EffectOptions.WaitForDoneOrCancel);
   app.DoMenuAndWait("Tools.ExtractRegions", false);

}

public void FromSoundForge(IScriptableApp app) {
   ForgeApp = app; //execution begins here
   app.SetStatusText(String.Format("Script '{0}' is running.", Script.Name));
   Begin(app);
   app.SetStatusText(String.Format("Script '{0}' is done.", Script.Name));
}
public static IScriptableApp ForgeApp = null;
public static void DPF(string sz) { ForgeApp.OutputText(sz); }
public static void DPF(string fmt, params object [] args) { ForgeApp.OutputText(String.Format(fmt, args)); }
} //EntryPoint