We are working on an update thus that Myst will run on iOS4. Of course, nothing can be as simple as only fixing the system volume function in question but we have to alter plenty of the app to produce sure it is compatible with 3.x as well as 4.0.
(Apple likes to make sure developers have plenty of (extra) work )
Hopefully, we should be ready to present an update to Apple by the end of the week, Hence depending on how rapidly Apple can examine it and acquire it on the app store (there may a lot of developers submitting updates for 4.0!), it might be a couple weeks earlier it is available. I'll hold you posted.
By the way , everything this is the like for Myst Free and Myst Japan. Myst Francais and Myst Deutsch appear to be all right in iOS4.