The word mantra is an early borrowing from Sanskrit via Hinduism into English, the donor language’s meaning being (according to the Oxford English Dictionary Online) ”A sacred text or passage, esp. one from the Vedas used as a prayer or incantation; a word or phrase from a sacred text repeated in this way. Also: a holy name, for inward meditation.” Its present-day occurrence, especially in the media, comes with the meaning (as defined in the OED): “A constantly or monotonously repeated phrase or sentence; a characteristic formula or refrain; a byword, slogan, or catchphrase.”
The traditional pronunciation renders the initial vowel as that of the garden-variety English word man, i.e. the flat vowel [a]. The ubiquitously erroneous pronunciation, heard constantly in the media, however, takes the word mantra as esoteric, hence marked (cf. my article in American Speech 72 (1997), 437-439), and identifies it with that of song, i.e. the broad vowel. This mispronunciation is clearly and directly the outcome of ignorance of the word’s traditional normative rendition in English, again due to imperfect learning.