Thanks for all the help Olaf. Everything's working now.
Ok, recap for anyone who'd like to add some more themes in the YR list:
-Convert files to wav format and place em in the RA2/YR dir. Converting them to 8.3 format would be better.
-Run XCC Mixer and from there go to Launch->XCC Theme Writer-> Red Alert 2 YR. The file thememd.ini has been added.
-Still in the Mixer, run XCC String Table Editor just like you did with the Theme Writer and choose YR again. In the new windows that appears select from the left side list THEME. Now highlight every new theme you recognize and edit it using the Edit button to what you'd like the name to be as viewed from the themes list ingame. You're done!
Again thanks for the help Olaf and maybe I'll be back pesterring you some more with other issues concerning slight YR modding.
BTW I heard you will be undertaking the task of cheaters fighting for YR from BA and Omerta sources. Is that true? Please elaborate a bit and also do visit the YR forums at BA.org to explain it some more for everyone there.