The finished piece, recorded or live, must achieve a good balance of properties, such as volume, pan positioning, and other effects, while resolving any arising frequency conflicts from various sound sources.
These sound sources can comprise the different musical instruments or vocals in a band or orchestra, dialogue or Foley in a film, and more.
The best mixing professionals typically have many years of experience and training with audio equipment, which has enabled them to master their craft.
A mixing engineer occupies a space between artist and scientist, whose skills are used to assess the harmonic structure of sound to enable them to fashion desired timbres.
Their job consists of balancing the relative impact of each audio stream, by putting them through effects processors, and having the right amount (dry/wet ratio) of each.