As the title states, the explosion sound associated with lightning plays no matter how far away the lightning strikes. A good way to test this is to create a ticking area at 0,0 in a superflat, and then fly or tp away about 2000 blocks. Run "/summon lightning_bolt 0 4 0" and listen as it sounds like the lightning struck directly next to you. This also causes a problem where other sound effects become cut off due to there being so many lightning sounds playing during thunderstorms.