Is Silent Prayer biblical and Does God hear and answer when we pray silently?

Is Silent Prayer biblical and Does God hear and answer when we pray silently? Search me, O God, and know my heart; test me and know my anxious thoughts. -Psalm 139:23

Yet you know me, O Lord; you see me and test my thoughts about you. Drag them off like sheep to be butchered! Set them apart for the day of slaughter! -Jeremiah 12:3

From an article on the subject:

The Bible may not specifically mention praying silently, but that does not mean it is any less valid than praying out loud. God can hear our thoughts just as easily as He can hear our words (Psalm 139:23; Jeremiah 12:3). Jesus knew the evil thoughts of the Pharisees (Matthew 12:24-26; Luke 11:7). Nothing we do, say, or think is hidden from God, who does not need to hear our words to know our thoughts. He has access to all prayers directed to Him, whether or not they are spoken.

The Bible does mention praying in private (Matthew 6:6). What would be the difference between praying out loud or silently if you are by yourself? There are some circumstances where only silent prayer is appropriate, e.g., praying for something between you and God only, praying for someone who is present, etc. There is not anything wrong with praying silently, as long as we are not doing it because we are embarrassed to be seen praying.

LINK to full article

If God can hear my thoughts and my heart about Him, then He can hear my thoughts and silent prayers TO Him. In addition, He has answered my silent prayers on more than just one occasion.

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Jesus answered and said to them, "This is the work of God, that you believe in Him whom He has sent." -John 6:29
Luke 17:21
"... the kingdom of God is within you."
Just because you are not saying the words, doesn't mean God can't hear.
Shall God not search this out? For he knows the secrets of the heart". Psalm 44:21 KJV
Another example that comes to mind is one from Jesus in Luke 18:11:
"The Pharisee stood and was praying this to himself: 'God, I thank You that I am not like other people: swindlers, unjust, adulterers, or even like this tax collector."



So a mute person cannot pray, interesting.
Romans 8:26 (New King James Version)

26 Likewise the Spirit also helps in our weaknesses. For we do not know what we should pray for as we ought, but the Spirit Himself makes intercession for us with groanings which cannot be uttered.

So if the Spirit prays for us, but we have to pray out loud, some would say what they are doing is not biblical because I have seen many a thread about people disagreeing with 'praying in the Spirit' in which people are groaning or crying out. So which is it?