What is the ruling on one who prays during a silent prayer with a mumble that can be heard but not understood? And is this mumbling considered an action?

Recitation in prayer, whether silent or audible, requires moving the tongue and lips. This is the minimum level of silent recitation, while the maximum is for the worshiper to hear only themselves. As for mumbling (tamtema), if it means making the person next to you hear in a silent prayer, this is a slight audibility and does not necessitate a prostration of forgetfulness (sujud sahwi). If mumbling (tamtema) refers to hastiness or rushing in recitation, there is no harm in it as long as it does not lead to a disruption of the recitation.