Then they can interact together and find their own holes to leave or fill in as their parts interact with others. Sometimes there is more than one stereo mix for the title . The drummer might want more bass and piano and the guitar player wants to hear a different mix with less piano, but more acoustic guitar etc. Next is another issue. Sometimes we hear a singer sounding good on one or several stages, and then one night they are off key, or singing poorly, or too soft or loud for the band sound. There is an issue for singers, Head Tone versus Room Tone. Head tone is what you hear mostly when you are singing outdoors or not close to a reflection source, like a wall or window. If you sing loudly at the wall, you hear a different tone then if you do not hear a reflection. People singing loudly in the shower are enjoying several, close reflection, room tones.