Mixing engineer

A mixing engineer (or simply mix engineer) is responsible for combining ("mixing") different sonic elements of an auditory piece into a complete rendition (also known as "final mix" or "mixdown"), whether in music, film, or any other content of auditory nature. 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 work is found in all modern music, though ease of use and access has now enabled many artists to mix and produce their own music with just a digital audio workstation and a computer.
Education.
Mixing engineers typically begin with formal training in a music background, namely a degree in audio engineering or recording engineering. Degrees in other relevant areas, such as those in music, or any working experiences gained outside academia can also help; for example, mixing engineers specialized in classical music may benefit from experience in performing in an orchestra to create better recordings. 
Two primary categories comprise the population of sound mixers: the live sound mixer, who practice live sound mixing, and the studio mixer, whose work is done inside a tuned studio instead. Gear and equipment may slightly differ between the two industries but, universally, a well-trained ear, practice on complex audio equipment, and a thorough grasp of the techniques used to create good sound distinguish the superior mixing engineer. These are traits gained through long-term practice and experience, within or without coursework.
Techniques.
Mixing engineers rely on their intuition in the process of mixing, but all mixers generally follow certain fundamental procedures:
Balancing a mix.
A mixer is given audio tracks of the individual recorded instruments to work with. They show up well after the artists or session musicians are done recording, and just have this audio to work with. 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.
Equipment.
Some equipment mixing engineers might use are: