var this_sound;
while(1)     //the existing while loop you mentioned
{
   ...
   if(player_proxb==1)   //if near player (?)
   {
      if(snd_playing(this_sound)==0)   //AND sound not already running
      {
         this_sound = snd_play(gear_wav, 100, 0);   //remember snd_play DOESNT self-repeat
      }
   }
   if(snd_playing(this_sound)==0)   
   {
      snd_stop(this_sound);   //stop it "properly" when its finished itself
   }
   ...
   wait(1);
}