Ok - I checked a bunch. You are correct, interesting, most of the commentaries I have are silent on this issue, but I have many and found at a least of few addressing the question.
Bible Knowledge Commentary - describes the voice as coming from an angel.
Holman Commentary just refers to it as a “heavenly voice” though notes other verses with a voice comes from the “the throne” (implication is that its not God’s voice)
Word Pictures in the new testament - says its not the voice of God, but of an “angel of the presence”
The Pulpit commentary suggests the proper gramatical construction is that the voice came not “from the throne” but the area of the throne - suggesting the source of the voice is unclear, but likely an elder.