We are all comparatively beginners and learning.
For any mantra, first get used to how each syllable sounds one at a time, then string them together in a rhythm that is comfortable for you to keep repeating. Getting used to that rhythm helps commit it all to memory.
While learning it and if you want to recite it quietly then you can lightly press your finger/s in your ears. When you do this and hum or whisper or even just breathe the words you will notice the sound fills your whole head, which also blocks out any other noise around you.
To vibrate it, you can make the sounds more pronounced with a lower tone that rumbles and you feel the vibrations in your chest and throat and you elongate each of the sounds keeping with the rhythm.
There is a thing called ‘the great voice’, which, I interpret as pretending to call out at great volume, intensity and depth, even though you are near silent.
It might take some practice but you can pretend to hear your voice as a deep rumble that vibrates out of you because it is the intention that is the important part.