How does the beginning of the play in both versions (Seneca and Euripidis) of Medea affect the audience's sympathy for Medea?
This is a good question. Euripides's version of the tragedy portrays Medea in a more sympathetic light than Seneca's version. Right from the beginning, the audience has a different feeling. Let me explain why.
First, in Euripides's work, Medea is seen as passive. She does not even speak of her plight. The nurse relays all of this information to the audience, while she is filled with sorrow offstage. She is, in short, seen as a victim, and people generally speaking have lots of sympathy for victims. In Seneca's version, Medea is filled with rage. She will take justice into her own hand. Medea is portrayed as a woman you do not cross. The audience at this point is not so much filled with sympathy but fear. The audience also gets the sense that this work is more about revenge.
Second, from the point of view of the gods, Euripides portrays Medea as an pawn of fate. In other words, what could she do? In Seneca's version, Medea takes control, does not show reverence towards the gods, indeed, she seems godlike herself. For this reason, sympathy is not something that comes to the fore.