Audio clips can be treated with a multitude of effects in Kdenlive, and the available audio effects are listed in this page.
Unless otherwise stated, the audio effects used in Kdenlive are based on LADSPA, an open-source architecture for sound plug-ins. They will be available only if MLT was compiled with support for LADSPA, and for achieving this the packages ladspa-sdk and libjack-dev and swh-plugins must be installed. However, this should not be an issue for people not interested in compiling Kdenlive locally.
Hides the audio from a clip. This is a MLT effect.
Adjust the variation of audio volume in a clip between two selected frames. This effect is provided by the MLT framework.
Remove from the soundtrack clicks caused by clipping, an artifact caused when the audio recording level was too high. If you can have a graphical visualization of the sound, in a software like Audacity, you will see the audio "overflowing" and having its peaks cut off --- and these peaks cause the clip artifact.
Emulate the sound of a vinyl record player.
Limit the audio volume.
A stereo equalizer, useful for adjusting playback levels separately for different tones, like treble, mid and bass.
Very commonly used by guitar players, this effect causes an inversion in the phase of a sound wave in regular periods of time. In a sound wave made from a constant tone, a phaser will make it sound like a "woouunnwooouunn...". Well, it is surely easier to understand it by hearing than by reading :)
Pitch Scaler 
Allows you to modify the tone modulation of a sound --- for example, to make a male voice sound like a female voice and vice-versa.
Pitch Shifter 
Rate Scaler 
Reverberation is what happens, for example, when you sing in a cathedral, by the sound reflecting in the walls.
Room Reverb 
Same effect as in reverb but with shorter delays, like in a bathroom.