Open your preferred audio production program, for example Pro Tools or Logic. Double-click the desktop icon. It may take a few seconds to fully load.
Open the session with the problem vocal. Click "File" and select "Open." Then browse for the session that you want to fix.
Play the track and observe the gain meter, the gauge that measures the volume, to identify the offending track. It will be the one with the highest gain meter reading. The gain meter is the small LED strip that "pumps," when sound is made.
Click on "Mixer." This opens a virtual channel display with slider dials. Each slider dial controls the overall gain, or volume, of a channel.
Hit "Play" so you can hear your adjustments in real-time.
Drag the slider dial down on the channel with the loud vocal. Start by bringing it to the level of the bass drum slider. If the vocal is still too loud, lower it some more until it is audible, but not dominating the mix.
Click "Effects" and select "Dynamics." From the drop-down menu, select "Compressor." A compressor limits the scope of the dynamic range in an audio recording. It does this by boosting the quiet frequencies and cutting the loud frequencies. This results in a smaller dynamic range across the audio track. This makes the entire audio sit better in the mix. It also makes a recording sound more "punchy" without making it louder.
Click "Threshold" and adjust the virtual dial. The threshold governs at which volume levels the compressor kicks in. You can set it high so that only the quietest and loudest signals are altered, or you can set it low so that most signals in the dynamic range are effected in some way.
Click "Ratio" and adjust the parameter. This setting governs the intensity of the compression. Set it to high so that the loudest signals are cut sufficiently. This does have some sonic side-effects though. A "ratio" setting that is too high can cause a "walled" sound with little or no dynamic space. Tweak the dial so you find a balance between cutting the peaks and retaining some dynamic space.