Thank you for your answer.
This happen for all files (wav, mp3…). I am using 1.10.20. I tried updating to 2.02.12 and problem is gone: the volume is louder. So, I think I should upgrade to 2.02.12. I didn’t upgrade yet because of this problem: Lag when calling FMOD_System_Init in 2.02.07 (vs 1.10.20). But I think I have to find a workaround.