The word "voice" refers to the speaker. AT: "The one I heard speak from heaven" or "The one who had spoken to me from heaven" (See: [[rc://en/ta/man/translate/figs-synecdoche]])
# I
The word "I" here refers to John.
# He said to me
"The angel said to me"
# bitter
"sour" or "acid." This refers to a bad taste from the stomach after eating something that is not good.