Sonadrin i noticed you're setting the stream during the Animation. Are you loading the file every time you shoot or at the beginning? Usually you can just load the stream in the object before hand. That could be it?
For what it's worth, as an audio guy, I prefer to sync my sounds in code (especially footsteps and such, which you can trigger on specific frames of 2d animation). I always find the AnimationPlayer a bit floaty for audio but then again, I'm still in Godot 3.x.