I'll help you out. Follow these steps carefully:
You've downloaded the IDX extractor map with all the files i assume, if so then copy the following file and put it in the map where you put the extract_sounds.bat and the xidx.exe file (Which should be C:/Program Files/Activision/Rome - Total War/data/sounds folder). Copy and paste the file pack_example_music_idx.bat in that map.
When you've done that right click on that same file and edit the file into the following if you ONLY want to edit the voice3.dat and idx file:
Code:
.\xidx.exe -cf Voice3.idx data\sounds\music\*
pause
Do the same if you want to edit ONLY one of the other files. You'll get it 
Then double click on the pack_example_music_idx.bat file and your new Voice3.idx and Voice3.dat files will be created.
Good luck