Mantras are only endowed with transformative electricity if offered in initiation (dīkṣa) within the mouth of a guru. It isn't so much accurate pronunciation, but somewhat the ability with which the mantra is endowed that offers it transforming capacity. Vishishtadvaita Vishishtadvaita is one of the most important branches of Vedanta, https://www.instagram.com/tantramantraaurvigyaan/