Mixing is an art – not a science. There are no rules to this game that cannot – and have not been – broken. Nevertheless, there are various aspects that most mix engineers generally agree on.
The single most important activity is balancing the levels of the individual instruments and voices against one another. I cannot stress how crucial this balancing is. A recording can be completely transformed when you adjust or re-adjust the balance between the instruments – for better or for worse.
If you are mixing a song, the lead vocal is usually going to be the most important element in the mix. Similarly, if it is an instrumental, the featured lead instrument – trumpet, guitar, saxophone or whatever – becomes the most important ...