Originally Posted by markun
Well, my guess is that the decrypted files all have the same values at those positions in the header, could be 00 00 00 00 ... or ff ff ff ff ... for example.
If that were the case there should probably be several more repeated patterns in the file. This is the case for example with the iriver Story HD firmware files which use a simple byte pad cipher, and the decoded file is a ZIP with an easy to bruteforce password. The Odyssey seems to be using a more sophisticated cipher. You might have to gain access to the system files and figure out the decryption routine from there...