The Sikkim Chamber of Commerce site (yes, Sikkim, India) carries a short piece on the meaning and use of mantras. After noting that “Every Mantra has six aspects: a rishi or Seer, a raga or melody, the Devatha or the presiding Deity, a Bija or seed sound, the Sakthi or power and a kilaka or pillar,” it cites HPB as an authority.
OM is a powerful, simple mantra which could be recited by anybody. Mantra Sastra or Mantra Scripture says that OM could be chanted in 170 different ways.. Madam Blavatsky has quoted some ancient Indian text that it can be chanted in 250 different intonations and that the siddhis or results attained by each method is different and good.
The rest of the piece can be seen here.