Hans writes, "I am a musician and I will practice with a group of musicians in an orchestra for two weeks. I am being paid for everything by my employer and the time spent in the US is only for rehearsals so that when this group visits my country, we can perform together. Can I go to US on a visitor visa or do I need another visa?"
Actually, using your B2 visa will be a violation of the terms (since you are not coming to visit or vacation). You need to decide this in consultation with the organization that you will be practicing with in America because they will have a better idea of what exactly you would be doing there. They can also provide you with supporting documentation to attach with your visa application. In my opinion, most likely this organization may recommend a P or a Q visa depending on the arrangement that they have with your group.