“Can I have everything louder than everything else?”— Ritchie Blackmore
Trying to get just the right volume for every element in the mix is frustrating, because there are so many factors contributing to how we perceive a sound. There is the timbre of the sound, the register it’s playing in, the part it’s playing, as well as its context (the other sounds and effects that surround and interact with it) and last but not least, your state of mind. I have often had the experience of turning a sound up for one reason, and then, a few moments later, turning it back down for another reason, which leads me back to where I began. This is why getting a mix together can be slow going.
The technique—if you can call it that—that has helped me the most with figuring out volume is to simply listen to the music as a totality, pay attention to my immediate impressions, and make adjustments based on that. Recently I made notes on some mixes. As I scribbled down shorthand for each piece (“piano quiet” and “overall quiet”) I realized that my task for the day was to listen through the pieces and track several sets of volumes. I needed to figure out if the piano was loud enough, if the supporting parts were loud enough (or too loud, which causes the piano to sound too soft), and also adjust the overall volume of the piece if need be. Listening through the beginning of the pieces, my immediate impression was that on most of them the piano lacked punch. It was too soft. But one of the pieces sounded better than the rest, so I used that as a benchmark in terms of volume, EQ, and compression settings. Then I went back to other pieces and tried to make them sound more like the one that sounded better than the rest.
Immediate impressions are valuable insofar as they tell you something is wrong with the music without explaining exactly why this is so. Your job is to experiment by making small adjustments until you can longer discern any problems in the mix and every sound doesn’t sound louder than everything else.