I don't know If I understood the question as it was meant, but:
1. the file Sh.sdl in the Sounds sub-folder configures the sounds associated with an event. You can modify it with the excellent software named S3D.
2. if you want to associate a sound to an order given via the graphic interface or key shortcut, this is hardcoded in one of the the *.act files in the game's main folders.
There are a number of commands that had associated sounds in SH3, but are devoid of them in SH4, like "Depth under keel is... meters"
AFAIK you cannot improve the situation in case no. 2, because no modder tackled the issue of modifying the *.act and *.dll files and there are no tools to change them. There was some debate around here, but the general understanding is that, while it is OK to change any files in the Data folder, those in the main folder are NO GO.
A recent reference about what is called "hacking the code" can be found in this thread:
http://www.subsim.com/radioroom/show...cking+the+code