I will try right now. As I use integrated vga, I will try with removing RAM.
I tried removing my RAM, and beeper worked! Abnormal POST was reported.
So I tried again to better understand. In the end => PROBLEM SOLVED
I'm 99% sure it's due to "QUICK POST" activated (you can choose this options in BIOS).
If you choose QUICK POST => ENABLED, your motherboard does not beep to say OK if there is no problem.
If you choose QUICK POST => DISABLED, your mobo will beep even to say "everything it's ok".
Thank you very much for your help in this matter.