The best I could come up with was 等他们的耳朵不响了, which just seems wrong and inadequate. I was wondering whether there was a specific term for it.
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searching ichacha with "ears ringing": i have a ringing in my ears 我耳朵嗡嗡作响,a drumming in the ears 耳鸣, ( iciba :...耳朵里(久久)地回响着),once Chinese text is available, for further usage examples search bkrs :耳朵嗡嗡作响:耳朵嗡嗡响|耳朵嗡鸣,耳鸣:many more examples:tinnitus; ringing in the ears; syrigmus; singing: 神经性耳鸣 nervous tinnitus 我耳鸣。 My ears are tingling (humming; ringing; singing). etc. – user6065 Feb 12 '18 at 5:58
"Ears ringing" translated into Chinese is "耳嗚"
https://zh.wikipedia.org/wiki/%E8%80%B3%E9%B8%A3
係指外在環境沒發出聲音卻聽見聲音的一種狀況 (hearing sound when there is no external source of such sound)
最常見的原因是因噪音型聽力損失 (most common cause of this impairment of hearing ability is loud noise )
“when their ears stopped ringing” could be translated as "等他们的耳嗚停止时"
But "耳嗚" is quite a formal (medical) and literary term. The more colloquial term for "ear ringing" is "耳中嗡嗡作响" therefore, your sentence could be translated as "等他们耳中的嗡嗡响声停下來时" (when the buzzing sound in their ears stopped)