רָנּ֨וּ שָׁמַ֜יִם כִּֽי־עָשָׂ֣ה יְהוָ֗ה הָרִ֙יעוּ֙ תַּחְתִּיּ֣וֹת אָ֔רֶץ פִּצְח֤וּ הָרִים֙ רִנָּ֔ה יַ֖עַר וְכָל־עֵ֣ץ בּ֑וֹ כִּֽי־גָאַ֤ל יְהוָה֙ יַֽעֲקֹ֔ב וּבְיִשְׂרָאֵ֖ל יִתְפָּאָֽר׃ (פ)
Ibn Ezra on Isaiah
Sing, 0 ye heavens, etc. This is a figurative expression ; it means: great rejoicing will be through Israel, for through the redemption of Israel the glory of God will be revealed to the whole world.