This tutorial assumes you already know how to set up sounds in an alias. If you don't, click HERE.
Open the ENTITY_BROWSER and place a SCRIPT_STRUCT in your map. This struct is going to be the origin of your sound.
Change the color of this struct to something easily identifiable for all of your sound structs.
We are now going to give this struct some new KVP's:
targetname - zm_mapname_ambient_sound script_label - looper script_sound - sound_alias_filename
The targetname honestly doesn't matter what you name it, but it needs to be something. To stay organized, I find it best to have it your mapname and then _ambient_sound so I know these are my 3D sounds.
You may now copy and paste this struct around everywhere and just change the script_sound to your new alias file.
Note that if you have too many sounds that are too close together you may glitch out your sounds in game.