Both sound mixing and audio mastering are part of the music production stages which help an artist and his/her record label to have a well-polished product or a contemporary competitive commodity to release to the market.

Sound mixing comes before the mastering stage, but it's done after songwriting (lyrics) and production (beat-making) have been completed i.e., once an instrumental has been made and the artist has already recorded his or her lead vocals including background vocal harmonies or ad-libs (usually in the case of Hip Hop artists).

The artistic purpose of mixing a song is mainly to create a nice balance (i.e. to adjust the volume of instruments relative to each other) and enhance frequency sweet spots of instruments whilst simultaneously attenuating unwanted frequencies that are either subjectively overbearing or not-so-pleasing to the ear. It's also the stage for pitch-correcting lead (and background vocals) of recording artists that can barely hold a note.

There are other numerous technical methods put into a task when mixing music such as manipulating the dynamic range of the instruments, creating a wider stereo image, etc. However, I am won't be going into detail about this stuff here, so let's leave it like that, shall we?

What happens after mixing?​

When a mixing engineer has finished mixing a record he/she will send that song back to a record company representative for approval. The standard practice for many sound mixers is to slap a brickwall mastering limiter for extra loudness so that it gets approved. If the mix is too quiet some people might assume the mix is bad yet that might not be the case at all. Hence, the aforementioned use of a digital audio maximizer plug-in.

If a song has been mixed by different multiple sound engineers, it's usually the record label staff that decides which mix will be given to a professional mastering engineer because most recording artists don't have an ear for a sound they can only sing or rap and that's about it. However, some do but that's a few of them.

Mastering stage​

Audio post-production (mastering) is otherwise known as the last stage of music production and it's not a stage of trying to polish a turd e.g. attempting to fix major issues like unintelligible vocals, audible distortion, too much reverberation, delays, etc. Nevertheless, it remains yet another essential technical process of music production although, in reality, it doesn't add much value in comparison to other stages except competitive loudness by cranking up the average levels as high as possible and a subjective second opinion about a mixdown (well, if you need one, that is).

Common myths about audio mastering​

Below here are misconceptions some people have about both audio mastering including re-mastering (i.e. when record companies decide to jack up the loudness of old vintage records and re-release them to the masses).

MythsReality
Mastering makes your song sound better.The illusion is louder and brighter and is thought to be better without proper gain matching. It's the same mistake made when people compare two different digital audio effects.

However, a boring song will remain just that, boring... Umm, next!

Note: on the contrary, (albeit it is a rare phenomenon) it sometimes doesn't take exceptional composition or a world-class vocal performance to end up with an incredible track since professionally executed sound engineering can make a song more ear-catching... sometimes but not always.

The idea though is to pair mediocre artists with top-notch producers and engineers who can deliver a specific sound not the jack of all trades who try to do everything but aren't experts in any area.
Mastering makes music sound good on all playback devices (speakers, earphones, and headphones).That's an outright lie.

It's a mixing engineer who has that responsibility because they have control of every single instrument or vocal in a project.

Accordingly, they can manipulate individual mono or stereo tracks in many ways a mastering engineer can't without ruining it with unnecessary audio processing.

Note: people never get to hear the original mix to compare the before and after, which is one of the reasons mastering is overrated.

Business-wise, however, this is good for mastering engineers because sometimes you can overcharge for only slapping a limiter on mixes that already sound good without offering clients a discount.
Mastering it's something you can't or shouldn't do on your own because you need a second set of ears.That's only if you don't know what you are doing or want to focus on one thing (e.g. recording, mixing, or producing songs)

There isn't a definitive right or wrong way to master a song besides the cardinal sin of distortion (hard-clipping), the use of time-based, modulation, and pitch audio effects unless it's done for a lo-fi aesthetic.

Note: if music could be approached objectively as some snake oil mastering engineers often like to arcanely put it out across in shady ways to seem all too important and irreplaceable in the music production value chain—best believe all critics, journalists, bloggers, producers, songwriters and lastly, musicians too would be doing something else working graveyard to make ends meet.

Of course, assembly line automation... ah, yes that sums it up.

All multinational record labels would've implemented that already.
Mastering gives you an objective perspective on your audio recordings.Absolutely no. Once again, I ain't gonna preach too long... well maybe. Otherwise, there is no such thing as objectivity in music. It's all a bunch of opinions, and personal biases arising from subjective perceptions, feelings, or imaginations.

For example, when you hire a mixing engineer you are paying them for their taste (or style) not because they are the only living being on earth with an undisputed scientific approach to coming up with a perfect mix.

The same applies to mastering music. It's a business of selling taste, not perfection. This is why plenty of recording artists always have their music mixed and mastered by the same engineers, album after album and single after single.

If you go somewhere else you are gonna get a new taste. It might be good, it might be bad—who knows?
You shouldn't mix or master music on headphones.No one cares, LOL.

Like literally nobody sits there while listening to music and wonder if it was produced, mixed, or mastered on headphones.

Hell, even if everything was done in a bedroom, if it slaps, it slaps. If it sounds nice it's going into a playlist, yasss! Well done mate, well done!
Is master buss processing mastering?Stereo, mix, or master buss processing is half-baked audio mastering.

We can disagree but that just means you are delusional. Here is why...

The moment you start fiddling back and forth with equalizers, multiband compressors, stereo wideners, and harmonic exciters on the master buss, you've crossed into audio mastering territory.

You are better off fixing things individually rather than being a sloth. Or, perhaps you might as well finish that half-baked process and crank the levels up! Why not?


So how do you get into this industry?​

In the days of old, you would start as a runner boy/girl and your job would be to make the seasoned veterans coffee or whatever.

Later on, they would invite you to the craft and start teaching you how the game goes, including the nature of the business too. All the ups and downs including all the trade secrets assuming you're a dedicated individual.

Summary​

Mixing music involves dealing with many pre-recorded audio tracks while for mastering music only one stereo audio track is needed in a session assuming the project at hand isn't an EP or LP album.

On average, it takes about 6 to 8 hours to mix an entire song from scratch depending on skill (or experience) and whether there aren't further mix revisions a client wants. As for mastering a record single, that's about 30 minutes on average because as the saying goes "study long, study wrong."
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