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I ended up agreeing to sing a cappella at a friend's wedding.
Literal
Friend [genitive-の] wedding-ceremony [at-に] a-cappella [with-で] sing [thing-こと] [into-に] became.
~ことになる ('it has been decided that X,' 'it's come about that X') is a frame for marking a decision or arrangement whose agent is implicit or collective — 'it turned out that I'll X,' 'it's been arranged that X.' Contrast with ~ことにする, which marks the speaker's own deliberate decision. The implication here is that the arrangement wasn't purely the speaker's choice — they were asked, or it was decided by circumstances. アカペラ is a loanword from Italian 'a cappella,' referring to unaccompanied vocal singing. A common wedding-performance style.