Opinions are often divided about matters like this in English and there is no correct answer accepted by all. Like you, I would go for was . CB
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"a concert or a ballet" as these are singular "it" was
If you said "a concert AND a ballet" this would be plural and "they" were - would instead be correct.
I would take out both words. "...a concert or ballet given hundreds of miles away" is tighter writing. I would also change "given" to "performed" but neither of these are the question. I would use "were" because both the concert and the ballet are being referred to as hundreds of miles away. The "or" only distinguishes that either could be listened to for the reaction to take place. To change