The sound equalizer is a feature that allows you to change the equalization of the environmental sounds (i.e. the sound of what is happening in the room you are, or a person on the other side of a table you are sitting at) amplified from your hearing aids.
To activate and make adjustments to the 'Sound Equalizer' please first tap the three dots in the bottom menu in the app > then select the Sound Equalizer button > make adjustments to low, mid and high tones