I think “silent Rosaries” are just Rosaries said in the heart or to yourself. God, Christ, and Mary and all the saints knows your heart and hears all even to yourself.
I pray the rosary walking each morning, if I were to pray out loud I would inhale a lot of dust, as it is usually pretty windy at that time, and so provoke an asthma attack.
The rosary is meant to be a combination of vocal and mental prayers. But it’s the love that our Lord and our Lady want. So praying silently is Ok. I often do it. I often do it while I listen to a scriptural rosary too.
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