Here's an Approach to getting it louder. I don't mind looking down at the dash to see if the turn signal indicators are blinking, but on at least our 2002 chassis, they're too low and too small.
I was going to do the mod noted here, then I decided to add truck turn signal lamps to our mirror arms. We have the tubular tripod type with plain metal mirror heads, not the single-arm Velvac mirrors. With the turn signal lights, it's plenty easy to see them blink in daylight so I didn't add the piezo buzzer.