"Mă aude vreunul dintre voi?"
Translation:Does any of you hear me?
Your English is incorrect. One would never say, "Does any of you hear me?" "Do any of you hear me?" is the correct version.
Is "vreunul" the definitive form of "vreun"? And if so, are we using the definitive form because it's the subject of the sentence?
Yes, "Do any of you hear me" would be the most common. Never " Does any of you..."