Regarding Isaiah 66:6 - Is "palace" an alternate understanding and if not, why would "the sound of the LORD rendering recompense to His enemies" come from the Temple (not from the palace)?
The prophet Isaiah (Yeshayahu, יְשַׁעְיָ֣הוּ) relates “A Sound from the Temple” ( ק֖וֹל מֵֽהֵיכָ֑ל ) to the Dedication of God. - In Isaiah 66:5 the purpose of the trumpet sound is to gather Yisraelites to the Temple courts to “Glorify YHVH” ( יִכְבַּ֣ד יְהֹוָ֔ה ) - In Isaiah 62:9, Yerushalem is being gathered for their Dedication to God : “But its gatherers shall eat it and they shall praise the Lord, and its gatherers shall drink it in My holy courts.” ( כִּ֚י מְאַסְפָיו֙ יֹֽאכְלֻ֔הוּ וְהִֽלְל֖וּ אֶת־יְהֹוָ֑ה וּמְקַבְּצָ֥יו יִשְׁתֻּ֖הוּ בְּחַצְר֥וֹת קָדְשִֽׁי)
Although one Sound ( קוֹל-אֶחָד ) might appear singular, this particular #Sound ( ק֖וֹל ) references the unison ( כְאֶחָד ) of multiple trumpets from the Temple, referencing 2 Chronicles 5:13 [MT] :
“And the trumpeters and the singers were as one, to make one sound, to praise and to thank the Lord, and when they raised a sound with trumpets and with cymbals and with the musical instruments and with praise to the Lord, "for He is good, because His kindness is eternal," and the House became full of the cloud of the House of the Lord.” ( וַיְהִי כְאֶחָד למחצצרים (לַמְחַצְּרִים) וְלַמְשֹׁרְרִים לְהַשְׁמִיעַ קוֹל-אֶחָד, לְהַלֵּל וּלְהֹדוֹת לַיהוָה, וּכְהָרִים קוֹל בַּחֲצֹצְרוֹת וּבִמְצִלְתַּיִם וּבִכְלֵי הַשִּׁיר וּבְהַלֵּל לַיהוָה כִּי טוֹב, כִּי לְעוֹלָם חַסְדּוֹ; וְהַבַּיִת מָלֵא עָנָן, בֵּית יְהוָה.)
The “Hekal” in Isaiah 66:6 refers to the Temple of YHVH ( הֵיכַ֣ל יְהֹוָ֔ה ).