Taken from the link posted earlier..
"Actually the magnet on the driver in a large, high-quality speaker is probably enough to screw up the data on a modern drive at 1cm or less, perhaps more. They are also made out of Neodynium and can be quite strong."
The general consensus from what I've seen is this:
While it may not hurt anything to have your case sitting so close to the enlclosure, if you personally are concerned about any aspect of the 2 units being that close, it is better to be safe than sorry.
1cm or less. As I said, this only cause a problem when the drive is moving and the magnet is placed extremely close to it. This is not because the magnet messes with the data on the platters, but rather it interferes with the armature in the read/write process.
As Squidge has said, there is no reason to fear magnetically. Vibrations....that's not going to HARM anything at all, as mechanical hard drives generally have a pretty high tolerance for vibrations and impacts. This is why mechanical drives are still fine inside laptops...which can be dropped and still function.
Either way, its perfectly safe.
Probably doesn't sound too great with the thin metal of your case vibrating with your music, movies, and games. Although I doubt you're too concerned about that anyway.