Andy,
Most of the flush-mounted speakers I've seen have been where Cam just suggested. It is possible that you could mount them in the coamings if there is enough space between the coaming and the hull topside.
For additional tips for this and other projects, check out the C22 site at catalina22.org. Click on links, then click on Skip Meisch's website. Then click on 'other upgrades' in the table of contents there. You can then click on 'Scott Sauve's upgrades' to find where he mounted his radio and the speakers he placed inside the cabin.
In the process of following these links, you'll find links to the sites of other C22 owners and the improvements they've made to their boats. It's amazing the things people have done to their C22s to make them more comfortable.

Check out 'ChipsAhoy' by Chip Ford, for example.
Good luck on this project and post pictures when you're done. I've been thinking about mounting speakers in my cockpit, too, but just haven't gotten around to it yet. Too many other projects on my plate.
Pat