Hello Jamgirl, welcome to Using English!
That's an interesting question. You "prick up your ears" when you hear something of interest; but I can't think of a similar phrase for "to listen out for something".
"Alert" has a visual origin; "attentive" derives from the Latin "attendere", which literally means "to stretch or strain towards", but probably in the sense of "stretching one's mind towards", rather than "straining one's ears".
So no, I can't think of an aural equivalent of "observant" or "vigilant"! Perhaps another member will do better...
Have a pleasant evening,
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