Around 1998 I started to use software to enhance the quality of audio recordings. By now I can highly recommend the software NCH WavePad for this purpose, as it contains what I consider to be the best noise filter (Spectral Subtraction) and the paid version has a multitude of features that massively simplify the analysis and specific modification of certain audio aspects.

Otherwise, for those who prefer not to spend money on audio editing software, I can recommend Audacity. In terms of functionality, Audacity and WavePad are quite similar, though there are some features in Audacity that WavePad doesn't have at all. However, some features that can be used for qualitative enhancement have more advanced algorithms in WavePad and therefore deliver better results.