You need to create sounds in the wav file format. It is pretty much exactly the same as the EL MOD process. You save and name all the files and then you build a script file that identifies which files should be played and when, Imagine each sound file is a chapter in a book. Now imagine the chapters were shuffled out of sequence. You would need an index to tell you what order to read the chapters. This is what the script file does. It has been a long time since I programmed anything for these, and I have forgotten the exact terminology.

The IBU2 manual has a section dedicated to this though.