I like your question... it is like a 'pin him in the corner' question and one I have never given much thought to until now, thank you. I guess first of all you have to think differently then in the terms of the five senses. The vocal cords work based on vibration if I am not mistaken, and that is what those in spirit are, energy that is vibrating... for that matter all of life is a vibration of energy, even you and I. How can a deaf person keep time to music? Through the vibration of the music they feel. Just because they don't hear the music and lyrics... doesn't mean it is not there, we the hearing know for a fact it is. When I say, "I hear those who have crossed over", I honestly do but not with my external ears, as a thought that has sound to it in my head. For the most part what I get are thoughts and images... and I also hear names and numbers. If I tell you to think of a cat... that is no problem... but if I tell you to think of Patrick, how could you possibly do that if your frame of reference does not have a Patrick in it. That is why I am able to hear certain things such as names and numbers.
Now having said that, I can tell you I believe that we all have heard sounds and voices from those in spirit... all I can tell you is that from spirit anything is possible... hope I answered your question.