Very sorry for the late response, my internet was out for a couple days.
Fist open your .soundscript file; it should be called GraveDigger.soundscript
In it you should find a bunch of numbers and words and crap like that. You want to find where it says GraveDiggerIdle.wav, GraveDiggerLow.wav, GraveDiggerMedium.wav, and GraveDiggerHigh.wav
Change those to the names of the sound files you downloaded.
•Note- make sure the sound files you downloaded are in the packs folder under the main rigs of rods folder.
If you do not wish to do all this work you can download my RaminatorSounds and rename in the .soundscript "Ram" and "RamStarter" to GraveDiggerEngine in place of Ram, and GraveDiggerStarter in place of RamStarter. Make sure to change the name of GraveDiggerEngine and GraveDiggerStarter in the GraveDigger.soundscript or else the new sounds will not work.
I hope this helps and if you need any further help, please don't hesitate to send me and Email or message on Sim-Monsters.
Have a Great day! :D