Route the vocal signal through the compressor. If using a computer to mix the audio, you’ll likely make use of a software compressor. If using a mixing board to mix the audio, you’ll likely make use of a hardware compressor. Since there are many different setups for mixing audio, this process will differ with each setup.
Adjust the threshold so that it is lower than the volume of the loud vocal phrases but higher than the volume of the soft vocal phrases. When the level of the vocal passes the threshold, the compressor will turn on and compress the loud vocal phrases.
Adjust the compression ratio to set your desired level of compression. A ratio of 2:1 will allow a 1 decibel volume increase for every 2 decibels the vocal volume goes above the threshold. A ratio of 6:1 will allow a 1 decibel volume increase for every 6 decibels the vocal volume goes above the threshold.
Choose a fast attack time for the compressor if you want it to turn on quickly and compress the vocal after the volume goes above the threshold. Choose a slow attack time for the compressor if you want it to delay before compressing the vocal after the volume goes above the threshold.
Choose a fast release time for the compressor if you want it to turn off quickly after the vocal volume has fallen below the threshold. Choose a slow release time for the compressor if you want it to delay before turning off after the vocal volume falls below the threshold.
Increase the output gain of the compressor to make up for all the volume lost through compression. In other words, if the vocal is compressed 6 decibels during its loudest point, then the output gain should be increased 6 decibels. This will bring the volume of the soft vocals to the front of the mix, while keeping the louder vocals at the same volume they were previously.