Q. As Shabbath Shirah approaches, we have learned that the Shirah should always be recited with great joy, as this promotes forgiveness of sins. Does one have to recite it that way in a mourners house or in the day of Tisha B’av? How about when the Avel himself is the Shliach Tzibur?

Thanks
　
A. Zohar (B’shalach and Terumah) and Midrash, quoted by many Poiskim, exalt the ones who recite Shiras Hayam with great joy, with detailed care, and with its cant illations. They are considered as if they experienced themselves the Exodus of Mitzraim and Hashem saves them and forgives all their sins. (Pri Chodosh, Mishna Berura 53, et. al.)

Because of the inherent requirement to recite the Shira with happiness, some Poiskim advise to refrain from saying it in a mourner’s house or on the 9 of Av (Tur O.C. 559, Bais Yosef and Darchei Moshe ibid.). However, the accepted tradition is to say it including when the avel himself leads.

Meiri (Taanis 31,a) after quoting the view of the ones who don’t recite Shiras Hayam, writes that he does not see any benefit in their opinion. Since all agree that the psalms of Pesukey Dezimra are said, similarly other terms of praise, inspiration and awakening for Tefilah can be equally recited.

Halacha (O.C. 223,3), states that two contradicting blessings, Sheheyanu, said in joy and Dayan Emes, said in mourning could be recited in the same occasion, as when a father dies and the son inherits his estate. We find a similar Medresh on Akeidas Ytzchok (Bereishis 22,11) that relates how tears were flowing from Avrohom Ovinus eyes, yet he performed the Akeida and followed Hashem’s decree with great happiness. An analogous aphorism in the Talmud (Brochos 54a) states, that one should bless Hashem for the dreadful things in life as he blesses for the good ones. The Talmud explains that it refers to accepting them all with joy. It is obvious that a person who is in mourning can still maintain a degree of joy in his hearth.

Horav Shlomo Miller Shlit’a explained that a person may be in joy for having had Yetziat Mitzraim in the past and be expecting, with joy too, the future redemption, yet he may be now in a sate of temporary mourning.

Rabbi A. Bartfeld as advised by Horav Shlomo Miller Shlit”a