it is not necessary to listen mantra from a guru by physical mouth.i refer vigyan bhairav and mahanirvan tantra where it is elabortated in detail about type of diksha and mantra power transmission in a disciple.for example -chakshusi diksha evoke mantra power just by a graceful look of master on disciple, sparsa diksha evokes energy by touch .One can see all these diksha takes help of one or another senses.Another category mentioned is pratibha jyanana which exposes swatantraya (complete independence) in inner and outer surface of conciousness,Sambhavai diksha takes help of MAHAR TATTAVA and results in direct bodha of mantra that is istta dev darsan (vision of diety of particular mantra) in SOMA mandala( nector or full moon state 16 kala)so here disciple even do not need to complete the purascharan of mantra he just get bodha of mantra.The swapana deeksha include vision of mantra in dream with out physical interaction of master.THERE ARE MANY MORE TO TELL now i would like to draw attention toward SUFISM ,here four sect is famous KADARIYA,CHISTIYA(USE MANTRA IN PHYSICAL STATE) NAQSBANDIAYA(USE MANTRA AT PRANIC AND OVERMENTAL STATE) SOHARVARDIA (USES MANTRA AS A VISION LIKE SURIJAN OR MANTRA DRASTA MUNI EXPLAINED IN VED USE TO DO)..........................so mantra is just like mother and she will not mind if you pull her hand OR climb in lap.........OM SHANTI