Optimizing performance with sound



I want to conserve as much memory as possible for optimal performance as the game has a lot of resources. The information provided for gm sound is quite vague too so I'm full of questions and unsure how to approach thing's.

If a sound is loaded into the exe (preloaded or not) does it stay in memory just like an external resource would if it's not deleted afterwards? Does playing more normal sounds at the same time hinder performance ? If so does using global variables to play and replace sounds work better than just using the sound_play function?
Is it necessary to use the sound discard or sound delete function after the sound stops playing (external sound or not). ?