In this letter Alain Daniélou explains that the power of a mantra doesn't depend on the secrecy of its syllables but it is only given by the initiation.
A very detailed letter sent by Daniélou to Yvan Amar on the theories concerning the Shabda (in hindi: word; in Sanskrit: word; verbal testimony). In this letter Daniélou mentions some articles he wrote on Shabda theory.