First Nephi 2:18-24
The softening of Nephi's heart in v.16 sounds similar to what most people experience when they pray sincerely about something. The Holy Spirit speaks to their heart, they receive an answer to their prayer, or a witness of the truth. That is how I know that the Book of Mormon is true, and the truth of many other things. Anyone else can know by the same power. Nephi went a step further. He was diligent and humble, and during his next sincere prayer, the Lord spoke to him.
This was different than the softening of the heart he had experienced earlier. This was a message specifically tailored for him. Whether he saw the Lord, or heard an actual voice or not we don't know - It was a revelation from the Lord through the Holy Spirit. Joseph Smith received many revelations where the voice of the Lord was not audible.
Nephi's first revelation would permanently headline every page of the scriptures for his posterity, and is a recurring theme throughout the pages of the Book of Mormon due to it's simple message.