Try other Sony Tape Deck files first. Sony uses the same IR codeset across devices of the same type. Only discrete codes might be different.
Since you found the MX5 file, I presume you may know something about codes.
The following link is a site to all Sony Codes. You can use the information on this site as input to the MakeHex program (Files Section) to generate a full set of codes.
[Link: hifi-remote.com]See this link from the NG forum where I instructed someone as to how to get a codeset for other sony tivo devices.
[Link: remotecentral.com]Lyndel