I put one in the flaps drive housing behind the seats. The speaker is almost useless during flight, but on the ground listening is ok. I also have an extra switch for the radio to switch it on without booting the whole plane.
RV-7 flying, IO-375 w. P-Mags, 200RV Prop
RV-8 flying, IO-375, P-Mags, 74RV Prop