en_tn_condensed/rev/10/04.md

but I heard a voice from heaven

The word "voice" refers to words spoken by someone other than the angel. Alternate translation: "but I heard someone speaking from heaven" (See: rc://en/ta/man/translate/figs-synecdoche)

Seal up

People in John's day would write a message on a scroll and then seal the scroll so that only the person whom the writers wanted to open it were able to open it. This is a metaphor for keeping something secret. Alternate translation: "Keep secret" (See: rc://en/ta/man/translate/figs-metaphor)