Mantra and Soul in terms of tangible energy levels are difficult to measure. Nevertheless, the presence of their energies can be experienced in terms of Auras.
To explain this statement, lets see a general occurrence in our daily lives. We often feel attracted towards some people and not so towards others. Ever wondered why?
It is because of the energies these people emit into their surroundings (as we all do). These emissions or the presence and levels of individual energies is an Aura, the higher the level of the energy of the soul the stronger the aura of the person, the stronger the aura the more its power to attract other energies.
The same hold true for the mantras. Mantras are nothing but the formula to call upon energies of the respective forms (often a mantra describes the form), enunciated in the form of equations in words using purity of phonetics. Hence it is said to have a clear pronunciation of mantras whenever reciting them.
When a person recites a mantra regularly the energies of that mantra bond with with the energies of the individual thereby increasing the aura of the person. Mantras are the derivatives of and from the “Nada-Brahma” and reach their potential levels only when recited. Once recited the energies realised is directly proportional to the energy levels of the individual. By this what I mean is the amount of energy derived from a mantra is dependant upon how much of it can be retained by the individual. The more retaining capacity of the individual( higher levels of personal (soul) energy), the more potent (energy giving) the mantra becomes.
So the loss of mantra energy is practically nil as we can only tap as much energy from a mantra as we are capable to use. The more we practice a certain mantra the more our levels of energies increase and again more we can get out of the mantra. The process is interdependent.
And even then if the entire energy of a mantra is not absorbed by the individual energy, it is dissipated in the surrounding thereby merging with the eternal Brahma itself who is the energy core which can neither created nor destroyed.