What's the meaning of すます in 耳をすませば?
How would you translate this?
澄ます without the kanji, and it means "to clear, to purify".
耳をすます【みみをすます】is a set phrase, listed in the dictionary in its complete form, meaning to listen carefully.
You can think of
すます, then, in this context, as meaning to clear out your ears so as to listen better.
It is difficult to translate
すます alone in this context.
耳をすます is an idiom meaning 'pay attention to any slight sound'
As stated above 耳をすます is an idiom that means 'to listen carefully'. If we translate literally there is a kanji for 'ears' - '耳' and a verb 'すます' which means - 'to clear' - so if we combine these it's - 'to clear ears' which is the same as 'to listen carefully'.
As for translation of 耳をすませば, it seems that すませば is a form of a verb to include 'if'. So I would translate it as 'if you/someone listen carefully' or something of that sort.