1st step: Copying sound from Red Alert2: Yuri's revenge
Open your XCC mixer, click on: Launch -> XCC sound editor -> RA2 Yuri's revenge.
Now a window will appear, with all sounds in Y'sR. Click on the sound you want to copy, than click on the extract button.
By default it'll be extracted to your RA2 directory. Close the sound editor, than search the extracted sound wirh XCC mixer.
You will see that its in WAW format, but TS using AUD format, simply right click on the sound and "copy as AUD" , so a copy
of the sound'll appear in the XCC directory - but in AUD format. You can rename the sounds, their name musn't be longer then eight characters.


2nd step: Adding the copyed sound to TS: Firestorm
Open you XCC mixer at the TS directory. Search for the Expand01.MIX, open that, than copy the Sound01.MIX.
Open your XCC MIX Editor, open the copyed Sound01.MIX, and insert your sounds. Don't forget to save before you close the window! Now open the Expand01.MIX from your TS directory. Delte the original Sound01.MIX and click on save. Than insert the Sound01.MIX you just modded. Save agin.

3d step: INI editing
Open the Sounds01.INI, and add the sound like this: 756=ASOUND. It's very important to add the same name in the INI as the sounds. So if you renamed the copyed sound to ASOUND be sure to define this in the INI files. If your voice is a unit's voice, like a selecting or attacking voice, you have to add one more thing to Sounds01.INI . Go lower in the Sounds01.INI and you will find a lot of sounds with this flag: priority= . If your voice is a unit voice add the next flage there :
(ASOUND) ; Do not use this signs () ! I'm sorry but I cant write the correct ones cus they are bind to my google toolbar
Priority=100

Save and close the sound01.INI , and put it in the TS directory.
Open the Firestrm.INI and add ASOUND to a unit.

Thanks, will it also work in regular Tiberian Sun this way?

I think it will. Just mod this files:

Rules.INI exept of Firestrm.INI
Sound.MIX exept of Sound01.MIX
Sounds.INI exept of Sounds01.INI
Tibsun.MIX exept of Expand01.MIX

Bah, still doesn't work for me. I have renamed the sounds APSOU1.AUD - APSOU16.AUD.

Renamed? If you did that, it still in WAW format. Use XCC mixer to copy them as AUD. (Read the tutorial!)