I keep leaving them on given that I need to watch the road and not the panel. The Civic Hybrid had a very loud signal -- any way to make the Prius signal louder?
I've had cars with annoyingly loud turn signal relays. Maybe that part can be swapped out, but you'd have to experiment...
How strange. My previous car was a 2000 Dodge Caravan Sport. I was always leaving the turn signals on. And that model even had a chime that would start to ring if you left the turn signals on for a full mile. But although the chime rang via the speakers, it did not get any louder when you cranked up the 10-speaker stereo, so it was easy to miss anyway. The indicator lights were tiny and easy to miss. But in the Prius, I never leave the turn signals on. I find it very easy to see the indicators, even while I'm watching the road.
I believe all you are hearing is the relay chattering. I suppose you could hook up a chime or buzzer to sound when the directionals are powered.