An ancient evil has been living off of the line of Sith Lords for a thousand generation. It caused the war between the sith and Jedi, the creation of clones, countless galactic conflict, the separatist, the Empire, the First Order... It created an abomination of the Force and destroyed entire planets just to spawn the true vergence in the Force so it can take control and be all powerful.

This being has survived by jumping from sith master to sith apprentice each time the apprentice became more powerful than the master. It took over Palpatine after Pals killed Plagues. It tried to jump into Vader after Anakin through the Emperor into the reactor shaft, but it was too late, for Vader was gone and ended up trapped within Vader's armor. It was this entity that was talking to Kylo thru Vader's helmet. It makes sense that it would have Palpatine voice, as that was the last being it inhabited.

This theory is probably wrong, but it's the best I can offer.