[BillP]

Just a shot in the dark:

Try a cell phone that has a conference mode. You could put the cell phone on the robot with a hands free attachment and wire the speaker leads into a small amplifier with a speaker. The person who is doing the voice would be on another cell phone and able to hear the conversation close to the robot (by virtue of the conference feature), and could then respond. The "voice" of the robot would then come from the amplified speaker and be as loud as you want it to me.