I agree 100%. Not that it must be soundless. You might shout from the top of your lungs - but to God only. Like on a mountain or in a forest, alone. If you're praying in a gathering of people, then let it be silent... Or so soft that nobody hears you. Let it reach the ears of God only, not other human beings. It's not for them to hear in the first place. It's very intimate. Like in the marriage bedroom, one whispers to their lover the most loving words just for them, not for Mr. Smith next door or for Mrs. Johnson across the street to join in hearing. That would have been very, very awkward and out of place.
God wants to be the only recipient of our prayers. Not share it with the whole world... If somebody overheard your prayer, but you didn't intend it, so be it, God will reward it the same way as if nobody heard it. We don't control all the circumstances, God knows it very well. He is just. He will certainly reward our best and most genuine effort.
But when a prayer is performed publicly, aloud or even silently in front of another person or people with the conscious intent of them hearing you or inferring that you're in prayer to God, it's a bad sin of hypocrisy and lack of true faith. Forbidden by Jesus!!!