Hi.
Audio correction:
The effects Gain and Volume (keyframable) ...
What is the difference between these?
Both change the amplitude of the sound wave.
And in this way (as a result) we get amplification of sound.

Perhaps Input Level and Output Level ? But who is who?
Could you tell me, please, explain?

And one more question.
Fade In and Fade Out (for audio)
These effects also change (in principle) the amplitude of the sound wave?
What category they can be attributed? Input Level and Output Level ?

Gain changes the volume over the whole clip and is applied evenly across the clip
Volume (keyframable) - also adjusts the volume of the clip but you can add keyframes and make the volume adjustments change to be different levels at different spots in the clip.

Fade in and Fade out also adjust the volume - up from silent or down to silent respectively.

These effects work on the output that you get when you render your project. They do not change the volume on the input source files.

These effects work on the output that you get when you render your project. They do not change the volume on the input source files.

This is strange.
In Adobe Premiere Pro
Gain generally refers to the input level or volume in clips. Volume generally refers to the output level or the volume in sequence clips or tracks.

Besides that:
http://sound.stackexchange.com/question ... ffectively
Gain' controls are typically used to get a good input level. So for example if you had a microphone with a very low output signal, you would boost the gain to get a better level into your mixing desk or audio interface. If you had a synthesizer plugged in you might need to turn the gain down to stop the signal distorting. Gain controls are often controlling an amplifier behind the scenes - i.e. your signal can 'gain' strength after going through the gain stage of your audio equipment. So gain is usually the first control in the signal path of your recording equipment.

'Volume' controls on the other hand tend to be at the end of the signal chain and are more likely to be attenuating rather than amplifying the signal. So on a mixing desk, having got a good level from each input at the gain stage, you use the volume faders to adjust the relative volumes of each channel to achieve a good mix. You are not typically amplifying those signals any more though.

http://www.allaboutcircuits.com/textboo ... fier-gain/

Because amplifiers have the ability to increase the magnitude of an input signal, it is useful to be able to rate an amplifier’s amplifying ability in terms of an output/input ratio. The technical term for an amplifier’s output/input magnitude ratio is gain.

I am sure you are right about the general usage of the term gain.
However, Kdenlive is not really an audio capturing tool. Sure there is the record monitor where you can capture audio and video but the gain effect does not influence what is captured here.
So the Gain effect really is just a volume effect on the output.