Matt 6:5
And when you pray, you are not to be like the hypocrites; for they love to stand and pray in the synagogues and on the street corners so that they will be seen by people. Truly I say to you, they have their reward in full.
This doesnt speak against spoken prayer, but it does speak to the heart of the matter, which is the condition of the heart "when" praying out loud. If we like to hear ourselves pray so that we can admire our own prayer, or that it is meant to impress others by its "spirituality", then our hearts are not right, and our answer may not come in the faith and humbleness that was lacking.