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.
"Ears ringing" translated into Chinese is "耳嗚"
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)