Back in the old days when the turn signal relay was a big metal cylinder under the dash you could buy "extra loud" versions. Unfortunately I dont think it's that simple anymore.
2 ideas that come to mind are what codebreaker suggested, an led tied into the turn signal circuits and an audible alarm wired in the same way. The simplest way to do this I can think of is to splice off the trailer wiring for left and right turn and connect those 2 wires together to the (signal) terminal of a standard 4 wire relay. Run your 12+ input and ground to the relay and use the 12+ out terminal to power your audible alarm inside the cab.
Edit, brain fart... Trailer circuit wont work since rear turn is also stop. The alarm would go off whenever you hit the brake lol. Need to tie into the left and right FRONT turn signals to run the signal terminal of the relay.
Edit again, I need to quit drinking.... Front wont work either, those are also the marker lights. Alarm would sound anytime the headlights are on lol. Need to tie directly off the turn signal switch!