Not sure if you can, maybe have a hunt around on BH Motorsports and see if there's a mod or ini hack to do it.
Best option though is to turn them off entirely and only use the regular spoken notes; you'll be much faster in the long run!
I'm not aware of such a mod, but as Damien said, you can turn them off for better visibility and realism. Just make sure to adjust the pacenote callout distance to get the audible pacenote calls at the best time.