You need ResEdit to change the file type. Or there are other File Type/Creator programs available as well.

In ResEdit, goto File>Get File/Folder Info then choose your file. You will see fields for Type and Creator near the top. For Soundjam, change them to MPG3 & SJAM. If you want to change them to something else, do the Get File... on a file that is like the one you want to change the corrupt file to and then copy the info.

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