yeah, neither matt 6:6 or philippians 4:6-7 explicitly say you must speak out loud but especially the matthew verse kinda implies this in the way it specifies requiring privacy

that also contradicts the common practice of preachers performance praying too, but not the speaking part

the speaking part is something i discovered myself, i'm sure there is more explicit indications of this somewhere in the bible though

i simply am in no doubt about it because of the experiences i had when i first started doing this, and getting answers both in action (people appearing to help me) and actually hearing words appear in my mind that were answers

anyway, YMMV probably but if you ever were thinking of doing something to switch up your prayer practice this is one i highly recomend