Inspect the LED display on the outboard, rack-mounted components. If you are using an equalizer to shape the sound, the fault could be located here. For example, the equalizer may have switched to "bypass" mode following a power fault. On the Berhinger Ultra-Curve EQ unit, when there is a power fault the signal bypasses the entire unit and goes direct to the amp. If you use the equalizer to boost certain frequencies, the lack of power in the equalizer could be the reason for the dip in volume.
Normalize the gain dials on the amplifier. Some amps have dual channels with dedicated gain dials. If you are routing a microphone to one channel, ensure that the gain dial for that channel is equal with all of the other channels.
Check the microphone cables. If the microphone cables are faulty, it can result in a diminished volume level. Set the gain on the microphone's channel to zero. Not doing so could cause a loud popping sound as you remove the cable. Unplug the XLR cable from the base of the microphone. Unplug the other end of the XLR cable from the XLR input on the sound system mixer console. Connect a brand new XLR cable and return the gain slider to the previous level. Test the microphone. If the sound is louder, this identifies the cable as the faulty component. If it doesn't, continue to troubleshoot the mixer and microphones.
Try the microphones on another channel. Slide the "master volume" slider dial to zero and unplug the XLR cable from the input jack on the mixing console. Put it in an empty input socket and adjust the channel settings for the new socket so that they are exactly the same as the previous input. It's important to match the settings so that you can accurately compare the output. Repeat for each microphone. If changing the input channel and cable doesn't work, the problem is likely to be located in the microphone, the mixer or the amplifier.
Adjust the compression settings. Turn the "threshold" dial down on the mixer compression settings. Compression controls the volume peaks of an audio signal. Its function is to prevent sound over a certain volume from reaching the amplifier. If the threshold setting is too high, it can overly limit the signal from the microphone, making it too quiet.