Powershell command for renaming

Hi there

I have a script I used to use in powershell for renaming file extensions from .m4a to .m4b, I've just tried to use it again but now it doesn't seem to want to work for some reason. The script was designed to look in include looking in sub-folders for files.

This is the scipt I have saved (someone kindly figured it out for me long ago):

It's quite possible I edited it accidentally and saved over the original resulting in the failing script above.

I'd be grateful if someone could show me what I've messed up or an alternative script which would do the same thing.

Thanks in advance!