The Intelligibility Filter capability consists of a "Clarity" filter which corrects the muffled sound effect obtained when down-sampling voice files and a "Boost" option which produces variable signal amplification in order to increase the perceived voice volume (energy) content of recorded files.
The Intelligibility Filter is an option which can be selected while doing any of the conversion operations.
The Intelligibility Filter parameters can be set in the program defaults. A "weak" clarity filter and "no boost" is usually the best choice, but it may require some trial and error to figure out what setting is best for your sound files.