Hello brother, and a happy new year!
Well what you are trying to achieve is simple, but it does come with some minor challenges...
First thing that needs to be clarified, is that some mods (like Titanium Wars) come is compressed format (sga file type).
This means that all its assets - including sounds - are zipped in one large file.
And this is the reason you do not see any directories in the Data folder of the mod.
Other mods, come in uncompressed format - that is, all their files appear at the appropriate places.
We used that format in older version of Unification, but the latest ones come in multiple SGAs.
Note that there are mods that use both SGAs and files - in this case the uncompressed files take precedence over the compressed ones in case of overlapping assets (files with identical names and paths).
What you must do:
1] Decompress the SGA file of the mod you want to modify.
2] Find the paths where the fdas in question are located.
3] Use your own fdas to replace those, by using identical names (effectively overwriting the existing ones).
Now, to decompress a mod you can either Use Corsix mod studio, or the SGA tool of the Tool Pack of DC.
So i tried to roughly copy and paste the files in the titanium wars data folders, inspired by the path in the unification wars folders (both _default.rat file and sound files)
(data/sound/races/spaces_marines/troops_weapons/heavy_bolter)
What happened : i have now a beeping sound when an heavy bolter is fired in Dark crusade.
Good thinking.
I suppose what went wrong is some missing rats (files that contain stats on how the fdas should be played).
In those files you can define max polyphony, pitch, volume based on distance of the camera, etc.
Look into one of the rats of any mod you may have on your drive, to understand how they work.
Note also that if you do not want to decompress the WHOLE mod - just to update a couple of sounds - you need to at least create the full path of the fdas you want to modify, ALONG WITH all the rats you find "in the way"!