Found how to do it....
For the record, I used :
http://www.minecraftforge.net/wiki/MrGReapers_tutorials/1.7.2_sound_tutorial
As the url lacks working examples, here is one :
The mod is "jlmod"
- You must create a sound folder \resources\assets\jlmod\sounds
- Inside, ogg files.
-> In this example, "sonar.ogg"
You must create an sounds.json file in \resources\assets\jlmod
Content :
{
"custom_sound": {"category": "ambient","sounds": [{"name": "sonar","stream": false}]}
}
You can then play the sound with something like :
world.playSound(posX, posY, posZ, "jlmod:mineral_detector_sound", 1.0F, 0.6F, false);