Our intention is to open-source all our code, but the graphics and sound will be copyrighted or protected since they're our own intellectual property.
Open source or not all work is copyrighted to the creator unless you sell the copyright, so you may be confusing things a little here. A copyright basically says I own everything about this legally and as such can re-issue different/new licences on it and so forth. Open source licences do not give away copyright but give away rights to code ownership, but not the ability to change the licence or anything not mentioned in it. Open sourcing is commendable but graphics and sounds also have "open source" licences so I'm surprised that you've not thought to use a Creative Commons licence except on things such as company logos and unique characters (latter debateable remember Codemaster's Dizzy?) to prevent plagiarism and so forth, but perhaps this is what you meant?
Some of our projects will be collaborative efforts with completely royalty-free graphics and sound too, so anyone would be welcome to modify them and contribute back. .
Cf. above, all the more reason for your company to give back those royalty free graphics and sounds.
The games will generally be free downloads, but with the option to donate back to our developers, and we'd like to set up a system where people who donate over a certain amount receive a boxed, printed copy of the game in the mail, giving collectors more incentive to donate a little extra.
Excellent approach IMHO - build up a user base, fans and content then gradually phase in a part paid for system. Me, I'm still stuck in the give stuff away phase
Best of luck
