Can't unlock encrypted external drive with filevault (osx)
Sadly you are going to have to get that volume unlocked before you can copy any data from it.  The encryption cannot be "cracked" in any reasonable period of time.

If there is corruption, the volume/data may be irretrievable.

Can you find a machine running Lion and try to unlock it one last time there?  If that works, copy all the data off the volume and onto a new volume.