Hi all.
It seems that there're many people that can't make the german voices to work, so here is my "guide" - I did this few days ago, after I get my U-boat add on and I don't have any problems or missing phrases till now.
1. Make a clean install of SH4
in German - selecting it from the first menu
2. Install the SH4 German voices patch
3. Go in .......
\Silent Hunter IV\Data\Sound and copy the folder
Speech any where you want - I prefer the desktop
4.Uninstall SH4
5. Install SH4 again but in English
6. Install the add on
7. Go in .......
\Silent Hunter IV\Data\Sound and delete the Speech directory
8. Move the already copied directory(from step 3) in .......
\Silent Hunter IV\Data\Sound
Note that you won't have english voices in the American campaign.
I hope that this will help you to solve this irritating problem.
P.S. If you want you can burn the german Speech folder on disk, so you don't need to do this every time you make a clean install
.