Great thanks Snoopy,

No more issue and you guessed absolutely the root cause.
As I changes the channels, I was fixed on CHannel 2 (because I didn't read precisely enough your answer). But this morning, I came back in the Midi settings menu and I found out the Voices/GM page. I believed I never went on this page but obviously I did. I delete CH1 on the first line and that's it.
I want to thank you again : I didn't used the right hand sound for several weeks because of that.

Now I know that this midi settings page exists, maybe there are useful features to be used. Could you explain to me what the roles of those settings and some examples of what could be interesting here ?

Regards - Patrick