The text tells us Jesus saw heaven open and the spirit descend (implying that only He saw it). The voice addresses Him directly as “son”; the “beloved” likely references the angel (LXX genesis 22) telling abraham that he has proven his fear of God by not sacraficing his (abraham’s) beloved son. So, not an angel, but either God or possibly Abraham (i.e. Jesus heralds replacement covenant bypassing contemporary priesthood and is condemned to be sacraficed where isaac was spared). 

Were it God Himself speaking we would have a scene where father, son and spirit all feature. Jesus is then immediately tempted by satan and helped by angels  – pretty intense baptism. 

^ interesting insight into Mark’s arc of Son of God affirmations.

Mark’s baptism scene seems rich in midrash.  After the baptism the descended spirit (immediately) takes Jesus into the wilderness  – this forty days compares to Elijah’s days in the wilderness of judah and Sinai (at the end of which God speaks directly to him and he goes to anoint elisha as a disciple). 

Preceding Elijah’s wilderness time is his contest with Baal’s prophets (a test of faith comparable to Abraham’s). Significantly, Elijah drenches his sacrafice in water (!) and following his success is empowered by the lord to hurry away. Of course, this is the same Elijah who prescribed Naaman to bathe in the Jordan (!) where he is healed and rejuvenated.