You are mistaken. The mtime is the time when the file was last modified. For a music track, this is likely when the file was created or copied. xmms should not be modifying the files when it reads them.

You should try looking at the access time (index 8 in stat array). It will be changed by xmms reading from the file. You might be able to make the assumption between the atime on the previous file and atime on current file is the play time for the current file. But you will have to verify this. It will depend on when xmms calls the plugin.