I made a script that fixes the problem.

1. Put an empty file to /home/ folder
2. name it for example "ets"
3. Open terminal and write "chmod +x ets"
4. Open "ets"-file you made with Gedit for example.
5. Write in file:

wget -q -O - http://nullkey.kapsi.fi/et-sdl-sound/et-sdl-sound.gz | gzip -d > et-sdl-sound && chmod a+x et-sdl-sound;
./et-sdl-sound;

and save the file.

6. You can now play Enemy Territory with sounds.
   Just write ./ets in your terminal.

THIS BUG DOESN'T APPEAR WITH EVERY LINUX MINT USER!
But if I helped someone, I'm glad I could help you :)