The sound of music

So you've decided you want to be an audio engineer. First, double check how serious you are. This is hard work. It requires continuous attention to detail. Much like an umpire you'll find that no-one ever comes and says "awesome sound dude." But anything that seems wrong to anyone will result in them coming to the sound man and complaining. So now you know that you are going to be blamed for everything and, in most cases, poorly paid. Ok, triple check that desire.....

So, how do you know what sounds good? Go listen to real, live, un-amplified music. That will tell you what real instruments sound like. Now go listen to hundreds of hours of well-recorded music. Finally, go find someone to mentor you. A good mentor will yield the best training you can get. Watch and listen to that person. They won't tell you everything they are doing as they are busy doing it. Ask questions.

A good mentor may start by asking you repeatedly what you hear. They aren't deaf. They are looking to see that you are observing which sounds are good and which aren't. They will also want you to start picking out instruments, voices, etc. Hearing them independently of the rest of the sound is going to be essential to making adjustments.

Here are a couple of tips that will help you be a better engineer:

1) Music should consist of the vocals and the band. It should not consist of the vocals............ and the band. It should be a coherent whole.
2) Look at each instrument or vocalist. Looking at them will focus your hearing on them. Do this for EVERY instrument or vocalist. 
3) Practice.
4) Try things. 
5) Recognize that since instruments are in different frequency ranges, they can be equally loud in the mix. But, if they are in the same
frequency range you may have to "move them around" to different frequencies or volumes to "duck" under the other voices or instruments.
This takes time and....
6) Practice
7) Get out of the booth and walk around. Most sound booths are in LOUSY locations. It will sound different in the space occupied by the
audience. Go back to the booth and adjust. Go back out and listen. Go back and adjust. Repeat often.
8) Recognize that you'll make mistakes. Outside of the studio that's real life. Most folks won't notice beyond a few seconds.

There's more of course....