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He brought the umbrella swishing down through the air to point at Dudley –– there was a flash of violet light, a sound like a firecracker, a sharp squeal, and the next second, Dudley was dancing on the spot with his hands clasped over his fat bottom, howling in pain. When he turned his back on them, Harry saw a curly pig's tail poking through a hole in his trousers.
(Harry Potter and the Sorcerer's Stone)
Why is there ‘was’ instead of the plural from, were?
When the semantic subject follows the verb and is an enumeration, English usually uses the closest-item agreement rule: the verb agrees with the first item in the enumeration.
There was a flash of violet light and a sound like a firecracker.
There was a flash of violet light and sounds like firecrackers.
There were flashes of violet light and a sound like a firecracker.
The third sentence does sound slightly less awkward than the second.
I don't think the plural would be ungrammatical, but it sounds awkward. Stylistically, the plural would emphasize the multiplicity of perceptible effects, which would not fit in this context where these are all effects of the same event happening all at once — they are aspects of a single phenomenon, which pushes for the singular.
If the effects were a grammatical subject, the verb would have to be plural.
A flash of violet light and a sound like a firecracker were emitted.
See also Mixing plural and singular list items with a single verb on our sister site (about a different but related case).
