Scripture seems to allow for both silent and audible prayers and I think it may be a misuse of Scripture to say that prayer must be done only one way or another. An excellent article on prayer which explores some of these considerations is "Prayer" by J.C. Ryle which can be found here

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“Men ought always to pray.” — Luke xviii. 1.
“I will that men pray everywhere.” — 1 Tim. ii. 8.


The Chestnut Mare