There are ancient Tantric teachings that say that everything in the Universe is vibration. And each vibration has a sound (even if it is in a frequency that we can't hear). The Universe itself began with sound, which then propagated and created everything. Even the Christian Bible begins with the popular phrase "In the beginning there was the Word", which -some will say - not intended to be understood as an actual specific word, but rather interpreted as that initial sound. Other schools of thought such as Buddhism also recognise the relevance of sound, and associate it with the esoteric work. Gnosis for example, says that in the moment of death our body produces a sound, and that sound will be the one to carry us to our next life. 


So specific sounds have specific frequencies, and we can connect to those frequencies. Let's say we were able to replicate the sound of unconditional love for example. That would mean that part of us (whether it is the voice, or the mind, or all of us) would start to vibrate in the same frequency as unconditional love, and we would therefore start to impersonate that state.


The idea behind mantras is also to be able to quiet the mind, which will help us to be in resonance with the frequencies that we aspire to incorporate.