You can add new sounds to the game by creating a sound folder directly under your main mod folder and placing your sounds inside.
Sounds have to be in the ogg format.
You can use vlc to convert a file to the .ogg format.
Simply open the file in vlc, right click it in the playlist and chose "Save...".
The Convert window will open. Give a name to the converted file by typing it in the destination field. You can also brows to a desired folder before typing the name. Be sure to include the ".ogg" extension.
In the Settings section, choose "Audio - vorbis (OGG)" then click start.
The conversion takes some time depending on the length of your sound file.
You have to create a new definition for your sound or the game will throw an error and you won't be able to use them.
So once you have an ogg file in your sounds folder, go to your script_add folder and open the sounds.ini file in a notepad (notepad++ or notepad2 are recomended to edit these files).
Create a new definition like this :
[sound_name] files=sound.ogg minVol=0.8 maxVol=1 bus=ingame
Replace sound_name with the name you want to use in the editor. It should be unique so that there aren't any conflicts with the main game sound names (you will be warned with an error anyway). To be sure, you can add a prefix like mod_soundName.
"files" is where you put the name of your sounds. You can add as many as you like by separating them with a coma, the game will pick one to play. This is usefull for varying repetitive sounds such as hurt or walk sounds.
minVol and maxVol define the volume the game will use to play the sound. The value will be chosen randomly between these two values.
bus defines wether the sound is played in the game : "ingame" , in the menu/inventory : "gui" or as an ambient sound effect : "ambient"
The next properties are optionnal and give you more control over the sound effect:
minFreq and maxFreq define the tone of the sound making it play lower (grave) or higher (acute).
randomizePosition starts the sound at a random position in the file length. Mostly used for ambient sound effects.
atnFactor1, atnFactor2 and atnFactor3 (this section needs details)