This is a thread I started a few weeks ago, it has a lot of really good information that you can read through and a lot of advice and exercises that you can do with your horse without over doing the barrel pattern and make him sour. If you have any other questions feel free to ask. Barrel Racing Exercises and Drills.
Having their teeth done regularly can make a big difference in your horse's attitude and willingness to work. When a horse is in any type of pain, you will most likely not get the results you want when training or showing and you could end up doing more damage than good for your horse.
As far as getting him used to the noise around the arena, especially in a show setting..it's just going to take time and seasoning.. You can do things at home, like playing the radio, or having flapping things or other noisy objects around the arena while riding him..but there isn't anything like actually being at a show and in that setting. It will just take him getting used to it without making it too much for him. Try to keep it fun at a show and keep his stress level as low as possible. You want a show to be a positive experience for both of you.