There is a file called "sounds.json" in "<content pack root>/assets/flansmod/". In that file there exist entries that specify each existing sound file, those entries have to be created manually (with gedit/notepad++/vim/etc). Only the sound files specified in there will work.
If you really got fps issues I would check if your world is FPS-heavy, if you have alot of mods installed, or if you have alot of packs installed. I run at 60 FPS stable with 131 planes, 206 vehicles and 207 guns.