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合唱コンクール、みんなソプラノとかアルトだったけど俺だけ…
→ “Everyone was a soprano or alto at the choir contest, but it was just me...”
合唱 gō¹sō¹ = choir (n.)
合 gō¹ = unite / join
唱 shō¹ = sing
コンクール konkūr¹u = contest (loanword from French 'concours')
、 = comma (punctuation)
みんな minna = everyone / all people (adv./pronoun)
ソプラノ supurano = soprano (musical term, loanword)
とか toka = and so on / for example (particle indicating examples)
アルト aruto = alto (musical term, loanword)
だった datta = was (past tense of 'to be' - informal/plain form)
けど kedo = but / however (conjunction, informal)
俺 ore = I / me (masculine pronoun, informal/casual)
だけ dake = only / just (adverbial particle)
… = ellipsis (punctuation indicating trailing off/suspense)