float vol = 1;

default
{
    state_entry()
    {
        // Start the Jetpack looped sound
        llLoopSound("67f5e4f0-0534-4d97-bc01-f297648d20e0", vol);
    }

    touch_start(integer n)
    {
        // Lower the volume of the sound, or go to full volume if already zero
        vol -= 0.125;
        if (vol < 0)
            vol = 1;
        llAdjustSoundVolume(vol);
    }
}