And how do we know for sure it is the creator's voice?
This is an excellent question. Some others had asked me same question, that what proof is there that Baha'u'llah is a manifestation of God.
In my view, I feel certain that there is absolute proof for sincere and intelligent seekers. However that requires spending some time investigating. I'd say at least few months of good investigation...but hey, at the end it is worth it.
From my view, the proof of Baha'u'llah may be divided into 7 categories generally speaking:
1. In several passages of Biblical prophecies two dates are given for appearance of two promised ones. Both dates match with appearance of the Bab, and Baha'u'llah, as well as the place of the city of that these promised ones appear match with the Bab and Baha'u'llah.
2. In Islamic prophecies the date is calculated for the appearance of Mahdi from Quran and Hadithes, and it matches with appearance of the Bab.
3. In Islamic Hadithes, the duration associated with Mahdi and Baha'u'llah's revelation matches with the duration of the Two. The place of the appearance of them also prophesied by Hadithes and Quranic verses matches with the Two Manifestations. Both name of Baha'u'llah and the Bab can be found in Islamic Prophecies as well as Figurative Biblical verses.
4. Both Baha'u'llah and the Bab did not have teachers and did not have access to Books, to learn things that they knew, yet they knew about everything as it can be seen from their Writings. The Bab was only 25 yrs old, a Persian merchant with virtually no education other than being trained as merchant, yet He wrote thousands of Arabic verses in the style of Quran, even though He was Persian.
5. The life of the Bab and Baha'u'llah are full of Miraculous events and signs that was reported by many of their companions.
6. Both Baha'u'llah and the Bab lived a righteous life, as can be seen in the recorded history.
7. Both the Bab and Baha'u'llah made many prophecies. Non of the prophecies ever failed, and many already are fulfilled.
Note that the Bab and Baha'u'llah lived just about 160 years ago and there is good amount of historical account about them, so, it can be verified that, for example at a given year/month where they were, and with who they were living and what they were doing. So, things can be verified. So, for example we can verify if they had teachers/ books, libraries or not. We can verify if they lived a righteous life or not. Their Writings are available with their own signatures, or stamps. So, it is not like Jesus or Muhammad who lived thousands of years ago with not much accurate historical accounts , and some had even denied their existence. Like I said, it takes at least some month to grasp of the above points if you choose to spend full-time days.