They didn't hear His voice when He was standing right beside him.
They certainly heard His physical voice when He was standing beside Him, but what they didn't realize was that God is spirit and it is the Spirit of God who gives life, not the sound waves and not the ink on pages. Those are both physical words. That is why He told them that those of God hear the voice of God, because God is spirit.
So tell me, where does that say we need to look outside of scripture to know what to do?
It tells you that all over the Scriptures. It basically comes down to the phrase "Hear, O Israel", or perhaps it shows up as His word in near you, in your heart and on your mouth. The Father explained it when He said, "This is my beloved Son with whom I and well-pleased LISTEN TO HIM" (Mat 17) It shows up in the phrase that His sheep hear His voice. It is explain in when it was written to all seven of the churches in Revelation 'Let those who have an ear to hear, hear what the Spirit has to say to the churches' It is covered in sooo soooo soooooo many ways. It is explained in when Paul asked the Galatians if they received the Spirit by works of the Law or by hearing with faith and latter that the Scriptures keep everyone under sin so that Jesus Christ might be revealed. So it is explain that every one who believes in Him (not the scriptures) shall have eternal life. He explain it to the Jews by telling them they were searching the Scriptures for eternal life instead of coming to Him. It was explained to Satan when he asked Jesus to make bread of those stones in the desert (obviously referring to the stones Moses brought down the mountain) so that Jesus responded that man must live by every word that proceeds out of the mouth of God, It is explained to the book to Hebrews who had grown up with the Scriptures when it was pointed out three times about "Today, If You Hear His Voice", and to encourage everyone today. So it is even point out to the Israelites that Moses spoke to before leading them out of the promise land when He called the Lord "I AM", because He has always been there to speak to us, even before the Scriptures were written. It was pointed out when we read the the word of the Lord came and spoke to Abram and Abram believed it was the Lord and that belief was counted to Him as righteousness, and that again was before the Scriptures were written. So it is even pointed out the various places where it is explain that with there ears they don't hear, and thus every time asked the people if they had ears to hear. He of course was talking about spiritual ears. So it was pointed out to Nicodemus when Jesus explained that you must be born again and that would mean hearing the sound of the wind.
Wow - I could go on and on, but you have the Bible and the writings that explain that you must hear His voice and that His words are spirit and those spiritual words are life to you.
Yet don't think because I write that you can not get to know Him personally. He said that He stands at the door and knocks and
if anyone hears His voice and opens the door He will come into them and eat with them and them with Him. Just seek Him and His voice, which is found on your heart and perhaps even on your lips at times, because His words are spirit. And yeah, test the spirits (see 1 Jn 4:1-3, but it is written that if you ask for bread is He going to give you those stone from the mountain that Satan has been saying to live by.
People - You can and the Lord wants you to get to know Him personally!! It is going to mean hearing with your spiritual ears, because God is spirit. It means find Jesus Christ and His words for you. It might and will take practice (See last verse in Heb 5) but begin today but "Today" is the specific day you are suppose to hear from Him.
Start of easy, because while it is easy to hear from Him it is not so easy to have faith in Him. So start by asking Him simple question you might be able to believe Him for an answer. In the healing Christian ministry I worked in we often had people start by asking Jesus Christ how He felt about them and then pay attention to what came to them in their spirit. Sometimes we started with asking Him if there was anyone they needed to forgive, because it is harder to hear from Him tell you He loves you if you don't know about His forgiveness, and understanding that comes with you forgiving others. I personally really figured it out while driving down the freeway and asking Him which lane I should be in. That wasn't the first time I heard Him but was how I really figured out that He is indeed always with us to answer our questions.
And my friends - He like you asking Him questions!! Because our faith blows in the wind, it is not always easy getting the answers, but He will answer them even if it takes a while to get through (See Daniel), and if they are easy to believe for questions then you will hear an answer immediately. It is written that He is not slow to answer like you think.
I could give you a lot more advice from listen to Him for about 20 years, but the real advice I have for
anyone is that they need to seek Him and His words for them personally.