The first thing you can do if you just want the voice gone is to delete the file. Open the folder SH3/data/Sound/Speech (or Speech_En if you use the English voices). You must then go into each of the nine folders and delete the file RaM_K_New_message_received. Better still, but a little more work, is to change one letter of the file title. This guarantees that the file is still there in case the program looks for it (it usually doesn't) and also lets you restore it if you should ever want to.
Another thing would be to look through all the download Speech files and replace the original with one you like better.
You can also download one of several full sound makeover mods people have put together over the years. This one is one I did almost a decade ago, converting the original Das Boot Sound Mod done for SH2 so it would work with SH3. It changes all the voice files to sounds from the movie.
http://www.subsim.com/radioroom/down...do=file&id=978