Insert a compressor onto the vocal track you want to level. Depending on your mixing setup, this can be done with a software or a hardware compressor. Since each mixing setup is unique, the process for inserting a compressor will vary.
Adjust the threshold control on the compressor so that its level is just above the softer vocal passages. This will allow the soft vocal passages to go uncompressed, while compressing anything louder than these passages. This is because when the vocal level passes the threshold, the compressor will turn on.
Adjust the compression ratio to control how much the vocals are compressed when they go above the threshold. A lower ratio of 3:1 will allow the volume of the vocals to increase one decibel for every three decibels the vocal level goes above the threshold. A higher ratio of 8:1 will allow the volume of the vocals to increase one decibel for every eight decibels the vocal level goes above the threshold.
Set an attack-and-release setting for the vocals. The attack setting will determine how quickly the compressor turns on and begins compressing the vocals after the level goes above the threshold. The release setting will determine how quickly the compressor turns off and stops compressing the vocals after the level goes below the threshold. A fast-attack setting will allow the compressor to turn on quickly after the threshold is crossed, whereas a slower attack setting will cause the compressor to delay momentarily before turning on. A fast-release setting will allow the compressor to turn off quickly after the level falls below the threshold, whereas a slow-release setting will cause the compressor to delay momentarily before turning off.
Turn the output gain up to make up for volume lost through compression. For instance, if the vocals are being compressed five decibels, increase the output gain by five decibels. This will effectively increase the volume of the softer vocal passages, without actually increasing the overall volume of the vocals.