I'm sure someone must have pointed this out before and I hate to seem like on of those noobs - but here goes:

The Accensi (Republican Light Reserves) have the unit fire sound of a bow and not slings.

I went to correct it and discovered you do not seem to have the file in the modfolder. So I copied descr_sounds_units_fire.txt from the vanilla RTW data folder into the EB data folder and added the "roman missile accensi" type at line 112.

The correct unit fire sound is now used in-game. You'll need to add in any other slingers to that file if you did not do so already and forgot to put the file in the modfolder!