I have a question about who is responsible, appart from the Project Manager, for determining the stakeholders´roles. I´m not sure if it´s only the Project Sponsor or any other stakeholder who can help in this activity.
It depends what you mean by "roles". Responsible for Stakeholder management is, in my understanding, the Project Manager, but he or she should get input from as many places as possible, as descriped in PMBOK 188.8.131.52, 184.108.40.206 or 220.127.116.11 on Expert Judgement
Moderators: Yolanda Mabutas, Ahmed Amin, Scott Gillard, Michael DeCicco, Rahul Kakkar, Fernando Jr Sinlao Lim, Mary Kathrine Padua, Chris Preziotti, Lazard Toe, ITBMC, MPM, CIPM, PMP, ITIL, Christian Winter, Kevin Nason, Michael Sumaquial, PMP, Mark Wuenscher, PMP