It really depends on what type of .XMI files they are. .XMI can be an extension for Everquest media files, XML Metadata Interchange files, or Extended MIDI music files.
If they are Everquest files, they would play using EQ's proprietary software. XML files should work in modern web browsers. Extended MIDI files should play back normally in pretty much any current media player. You won't be able to convert them, but they should play normally in WinAmp and Windows Media Player.