if(snd_playing(engine_thrust) == 0)
{					
 snd_play(engine_thrust,50,0);
}

SOUND* engine_thrust = "some_sound.ogg";//this is not the actual filename

var snd_handle = 0; // global variable
...
if(snd_playing(snd_handle) == 0)
{					
 snd_handle = snd_play(engine_thrust,50,0);
}