How does Oneness Pentecostalism interpret Matthew 3:17? Matthew 3:16-17 NIV

16 As soon as Jesus was baptized, he went up out of the water. At that moment heaven was opened, and he saw the Spirit of God descending like a dove and alighting on him.
17 And a voice from heaven said, “This is my Son, whom I love; with him I am well pleased.”

If there is no distinction between the Father and the Son, as Oneness adherents believe, how do they explain the voice from heaven?
Interestingly, Oneness Pentecostals do not fail to see a distinction between Father and Son as many suppose.  The United Pentecostals explain the distinction of Father, Son and Holy Spirit as follows: "God is Father in creation, Son in redemption, and Holy Spirit in sanctification."  Note however, that the distinction is in what God is doing and how he is doing it, not in person-hood.

The doctrine of Oneness denotes that there is only one God who manifests/reveals/expresses Himself at various times in various ways. This is in contrast to Trinitarian doctrine which sees God as always having existed as three separate and distinct persons, though one God.  

One United Pentecostal theologian, David Bernard, in his book, the Oneness of God, points out that Scripture uses the term Son when referring to Jesus' humanity, never His divinity.  Thus, they see a distinction between Father and Son in terms of humanity.  Jesus the Son says He is one with the Father, in the Father and the Father is in Him.  They therefore see one God existing simultaneously on Earth in the Son and in heaven as the Father.

Having established that background, in Matthew 3:16-17, God is in the Son, who is getting baptized.  God speaks from heaven as Father, addressing His Son.  God also alights upon  the Son, in the likeness of a dove.  But He is only one God simultaneously in three places, doing three things, in three different ways.

Bernard writes:


  With the
  omnipresence of God in mind we can understand the baptism of Christ
  very easily. It was not at all difficult for the Spirit of Jesus to
  speak from heaven and to send a manifestation of His Spirit in the
  form of a dove even while His human body was in the Jordan River.  The
  voice and the dove do not represent separate persons any  more than the
  voice of God from Sinai indicates that the  mountain was a separate
  intelligent person in the Godhead.


The rest of what he writes in regard to Jesus baptism is found on page 172-175 of his book, The Oneness of God.

There are various Oneness adherents, and even various Pentecostal Oneness groups.  I have answered based on the doctrines of the United Pentecostal Church. 

See also my answer to a related question:
How do United Pentecostals and other Oneness groups interpret Hebrews 2:9?