What is the significance of the voice from above the Raki'a?
1.
Rashi citing Targum Yonasan: When Hashem desires to make His word known to His servants, the Nevi'im, a voice is heard from above the Raki'a, from between the Keruvim under the Raki'a above their heads.
2.
Malbim: This is a fourth situation of Chayos' effect on Ofanim. When Hashem conducts His world alone - "Hashem ba'Shamayim Hechin Kis'o" (Tehilim 103:19), and not via the Chayos, He puts His voice on Shamayim and the Ofanim. Then, the Chayos cannot do anything. Then they act via the Serafim in Olam ha'Kisei, like Yeshayah saw, or without any intermediary at all, like at Yetzi'as Mitzrayim and in the Midbar. At such a time, the voice is above the Raki'a over [the Chayos], from the Kisei or the supreme Atzeilus.
2)
Why do their wings soften?
1.
Rashi: It is due to fear of [Hashem's ] speech.
2.
Malbim: They cease to act (move the Ofanim), and their wings do not receive influence from above.
