Presumably, it’s God the Father in heaven whose voice is heard. It seems to be a single-mediation communication. You say that God always speaks through his Word, but God also speaks one-to-one without mediators, and he also speaks through his Spirit.
Therefore, the question (at least to me) becomes:
Who is the heavenly voice describing? I think it can be seen as the Holy Spirit who is descending on Jesus. The traditional interpretation is that God is telling Jesus and/or the crowd that Jesus has won divine favor and is being rewarded by a public proclamation of his sonship.
I don’t dispute that, but I see no good reason not to also imagine that the heavenly voice was *also * refering to the “Spirit Like A Dove” who at that moment was descending, and would soon “roost” (Luke), on Jesus.
And why not? Inasmuch as Jesus’ speaking (and teaching) can be said to not only be inspired by the Spirit, but to actually be self-revelations of, and by, the Spirit, we can say that the Spirit, like Jesus, has a “son-like” role in his ministry. The Gospels support this:
Both Jesus’ friends and foes agreed that Jesus “had” - possessed, or was possessed by - a spirit. His foes called that spirit “Beelzebul”. His friends called it “a” - or “the” - Holy Spirit.
This spirit “drives” Jesus (Mark) into the wilderness for a typically “shamanic” ordeal of prayer and fasting, wherein he battles and defeats another, evil spirit (the Devil).
By this holy spirit, Jesus drives out other “unclean” spirits. His enemies take note of this, accusing Jesus of “having” an evil spirit. Jesus replies with the famous “a house divided against itself cannot stand”.
Never does Jesus deny that he “has” a holy spirit - in fact, in one famous text, Jesus is depicted as “rejoicing in the Spirit” … and at one point he insists that the only unforgivable sin is the one “against the Holy Spirit”.
Therefore, functionally, the Spirit, too, has taken on a “son-like”, Messianic role. Jesus’ spirit guides him and enables him to prophesy. Both Jesus and the Spirit are in league, establishing God’s Kingdom, in tandem, on earth.
If this seems a bit far-fetched: just try to imagine Jesus’ ministry without the Holy Spirit; try to imagine the Apostolic ministry without the Holy Spirit; try to imagine Pentecost… well, you see where that goes.
God the Trinitarian Son is incarnate in Jesus’ human “vehicle”. God the Trinitarian Spirit descends to assist the *human * Son, thus functioning as a kind of auxiliary “son” - and a kind of “brother” in the furtherance of Jesus’ work and teaching.
I don’t find it strange at all that God would grant “Sonship” to the descending Spirit - IF we limit that role to a temporary earthly function designed to assist Jesus’ ministry.
So the Gospels’ scene of the Baptism/Descent of the Spirit could be seen as the Father in heaven publicly approving and identifying Jesus as the Son on earth … and also designating a “son-like” function to the Holy Spirit as a seal on Jesus’ ministry.
