I am trying to make a clean job in Ardour about the volume of the exported file, but it seems I made some mistake, can someone help me ?
I did this test : I put only one MIDI track with a Surge XT patch (Kick, from Argitoth) in Ardour, I didn’t touch track or master level (which turns seriously red) and did my normalized export.
The fact is that the volume of the exported file is very low.
So, this is because your loudest peak is as loud as it can be, but that peak is very brief. You are either going to need to use different samples, or start working with compression etc. so that there isn’t as much a difference between the peaks and the majority of content.
I also see in the spectrogram that the majority of the energy is below 100Hz. The loudness of a sound like that will vary considerably depending on the low frequency capabilities of the monitoring speakers.
I suggest to add a limiter to the master-bus. I can offer x42 Digital Peak Limiter (also packaged by most GNU/Linux distros), should be fine even with default settings, or as @seablade suggested add a compressor to the track, after the synth.