Your words will be more powerful if it is pronounced aloud. Just remember how effective it is to read aloud than to read in sleeve. If you pronounce a word definitely it stores into your subconscious mind more certainly. Similarly, if you pray God with sound, there will be certainly difference in the power of prayers upon you.
National Discovery does not promote superstitious beliefs. If there is really the thing like Mantra, it must be the power of word which is one of the part of Psychology. Hope you get some ideas about Mantra in this article.