Place a sound and this script in an object and it will play the sound looped.

The number "0.90" in the script is the volume and can be changed to any number between 0 and 1. "1" is full volume.

 

 

default
{
    state_entry()
    {
        llStopSound();
        llSleep(.2);
        llLoopSound(llGetInventoryName(INVENTORY_SOUND, 0), 0.90);
    }
}