I have a 2-week old mule at present (and growing rapidly), and he makes the weirdest noises when he tries to reply to his mother's neighs. It's like he wants to copy her, but he can't (bc he's got different vocal chords) and he makes these groaning noises. Nothing like a donkey, although it may be technically a bray.
With regards to your novel, my husband says that mules are largely silent animals, so if your fictitious mule just pricked up his (considerable) ears and broke into a trot, I reckon that would be fine, too.
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