A simple question that I can't find an final reply for:

Why every sample in Ableton Live starts with -12 dB?

uhh . . . when you audition them in the browser, they are much quieter than when placed in a clip slot . . .

That is the default for simpler/sampler. You can change it by creating a simpler/sampler instance, change the volume to what you want, then save that preset in your library in defaults/Dropping Samples/ then put the preset in either "On Drum Rack" or "On Track View" folders depending on which you want to change

its also nice to add a default velocity value since by default velocity does not affect samples either

Just as a random note, a lot of samplers do this by default. Usually samples from sample packs and what have you set all their samples to hit 0 db, to use up all the real estate of the range. So when you make a drum loop or something, you wouldn't have every sample hitting that 0 dB, it would clip everywhere and just be too loud, so hence you would turn everything down so when everything plays at once the meters aren't going red.

So it mean that the default -12dB for samples in Ableton Live is just a way to help you to mix the track preventing clipping?

or not?

That's normal but for Example in Kontakt you have option to choose default startup volume.

but anyway you can use empty presets, save it with -6 db or 0 db for example to just load empty sampler with drag and drop with already 0fb set on it..

If you are using Simpler then there is right click option on Volume (windows) and then there is "Copy value to siblings" which will make all Other simpler's in your project set up to the same volume with one click..