This is where trained ears and cutting-edge technology marry to produce dramatic results. Where the goal is to maximize intelligibility of speech, we achieve this in a multi-step process. Once your recorded evidence has been cloned (we never apply processing to the source file), we sample the background noise floor and minimize it. Ambient noise, traffic rumble, wind, hum, once these components are reduced, we are already far along in cleaning up the recording. Next we attack paper rustling, car horns, any sort of non-vocal transients; these can also be greatly reduced using special software. Yet another type of software is then applied to "focus" the voice component, that is, to reduce echoes and reverberation, which tend to obscure clear speech. The next step is the enhancement of vocal frequencies to the exclusion of those above and below the voice spectrum, and finally we apply compression to boost low-level or quiet speech. The result is a marked improvement in intelligibility.
This enhancement process is not a 'paint-by-numbers' affair. The incorrect or excessive application of processing at any step can actually worsen the fidelity of the voice portion of the recording. It takes the discerning ear of a trained audio professional to carefully apply the various processes in the right order and in the proper manner to coax the recording along toward maximum clarity.