Yevamos, Chapter 14, Mishnah 8. The Mishnah opens by setting out the case: "Shnei achim pik'chim nesu'im lishtei nachriyos, achas pikachas ve'achas chereshes" - two brothers of full mental capacity who are married to two women unrelated to one another: one of sound faculties and one a deaf-mute. It turns out that one of these marriages is valid by Torah law, and the other only by rabbinic enactment.
The brother married to the deaf-mute dies:
"Meis pikeach ba'al chereshes, mah ya'aseh pikeach ba'al pikachas? Koneis" - when the brother who was married to the deaf-mute dies, she falls to yibum before his brother, who is married to a woman of sound faculties. The Mishnah rules that he takes her in marriage, that is, he performs yibum, because the option of chalitzah is not open to him: a deaf-mute woman is not fit for chalitzah and cannot perform it, and therefore he is obligated to perform yibum.
"Im ratzah lehotzi - yotzi" - after the yibum he may give her a get. Since he is of full capacity, his get is valid by Torah law, and it has the power to sever the bond that was created. This is certainly so here, since the bond created through him is only rabbinic; but even if the bond were of Torah law, he would be able to dissolve it, because he is of entirely sound faculties.
The brother married to the woman of sound faculties dies:
"Meis pikeach ba'al pikachas, mah ya'aseh pikeach ba'al chereshes?" - here the brother who was married to the woman of sound faculties dies, and the yavam is the brother married to the deaf-mute. His own marriage to the deaf-mute is of no consequence for our discussion at all; what matters is that the yavam is of full capacity and his deceased brother was as well. True, the deceased brother's marriage was valid by Torah law while his own is only rabbinic, but that makes no difference in this case.
The Mishnah therefore rules: "O choleitz o meyabeim" - both options are open to him:
Chalitzah: the yevamah is of sound faculties and fully able to act, and so is the yavam, and therefore the chalitzah is valid.
Yibum: he is permitted to perform yibum with her.
In summary: in this Mishnah we dealt with two brothers of full capacity, one married to a woman of sound faculties and the other to a deaf-mute. When the brother married to the deaf-mute dies, the yavam may only perform yibum, since a deaf-mute woman is not fit for chalitzah, and if he then wishes to release her with a get, he may do so. And when the brother married to the woman of sound faculties dies, both paths stand equally open before the yavam: he either performs chalitzah or performs yibum.