Frank,
I had a few more thoughts and sure don't want you to go stir crazy talking to yourself
I'm sure Stu or any ham can set us straight on what frequencies can and cannot be transmitted on.
Many years ago, the last time I went to the drive-in movies they had a setup where you clipped something to your antenna and tuned to a specific freq to get the sound. Something like that might work. I don't think a section of wire will be a problem for guys like me that buy cable ties by the carton.
[edit after re-reading your last post.]
The mike in a section of vacuum tube sounds like a good idea for keeping it directional.
Overall, it sounds like a great idea. Much better than me hanging over the side with one end of a 4 foot piece of vacuum tube taped in my ear and the other end zip tied to a broom stick while someone else drives over bumps.
- Dan