Are women allowed to give a speech when their audience is 50% men as well?
This is not proper for a religious crowd.  Poskim advise against this. Some even forbid it outright. 
However, if the crowd is not religious, and the purpose is to teach them Torah, there could be some room for leniency in very specific scenarios, and one should seek guidance from a Rav. 
 Men need to look at the speaker, and to have men looking at a woman speak for a long period of time is not Tznius. We find in the Poskim that by the age of 9 it is important to have only male teachers.
 See Shaarei Yosher III: 21. Shevet Halevi 3:14. Mishnah Halachos 5:218. 11: 58. L’horos Nasan 1 EH 60:5. Igros Moshe OC 5: 12. See also ibid. 2:36.
 See also the end of question #2806 (2).