In fact, prayers can be made silently and completely without outward manifestation
Yep, not even an audible "Amen" is required.
When a Christian representing the congregation in prayer concludes with this expression, those listening may also add an “Amen” in their hearts or audibly to indicate their strong approval of what has just been said.—1 Corinthians 14:16.
The Watchtower 2006, September 1, page 28 par. 7