the Looping parameter in the Play Sound node means how the modifier updates with relation to the time code not the song itself.
We understand how this can been confusing and not wanting to just play a sound once . The reason that you can not do this is Notch yet is because Notch does not have a sound engine. However I have sent the request to the dev team.
Depending on what you are trying to achieve in your project you can always kill the sound via using the Sound modifier node. this will then allow your Play Sound node to still be in use if it is being used to trigger other animations within your Notch project.
Definitely place where Notch can improve. Some more sound nodes maybe even an oscillator that can be manipulated by modifiers and maybe even some audio effects would be amazing.
Anyways my work around to get a single shot of audio is simply turn off looping and automate/key frame the volume to just fade out after the file plays once
Thanks for the suggestions. There is work underway to overhaul a few of the key sound features in Notch, including introducing more flexible playback options as well as easier ways to react to real-time audio.