Could there have already been a resurrection and gathering of the dead in the universe, especially if the questioner clearly hears the voices of their deceased parents but cannot communicate with them? And does this contradict the fact that the first to have the earth split open for him will be the Prophet Muhammad, may Allah's prayers and peace be upon him?

There will be a general resurrection for all the dead after the blowing of the Trumpet, and the Prophet Muhammad, peace and blessings be upon him, will be the first to be resurrected. This has not yet occurred. As for the special resurrection or bringing back to life of some individuals, it has happened, and it is contrary to the general rule, for the original principle is that the dead do not return to this world, as in the hadith (definition) of Jabir with his father. However, Allah the Almighty has made exceptions for certain individuals whom He brought back to life after their death in this world, for the wisdom of demonstrating His power to resurrect after death, such as those mentioned in Surah (definition) Al-Baqarah, ayah 243. What the questioner mentioned in her question is either a misapprehension or from the whispers of Satan.