The iOS does not have true multitasking. Only certain features of apps can stay active in the background, such as iPod music. But this requires that the currently active app is not using audio. Many games include their own music, so it cuts of the background music, as iOS only allows one active audio source at the time.
You will notice, that the iPod app also stops playing, when opening the Youtube app, or even Skype.
Anyway, what this means for the user is, that it is not possible to have the iPod app running in the background during games, unless the game supports this. If freecell doesn't have an option for this under its settings, then there is little you can do to prevent this. A jailbreak with Backgrounder might be a workaround. though only for iPad 1.