I believe even the earliest client had the same format: mp3 files, and a config.txt file which maps regions to music files. If you can pick up some older clients you should be able to copy the mp3 files over, and either rename them, or adjust the config.txt file accordingly.
Yes those are great files they sound so much smoother than the first midi files. Thank you for posting this ! I was looking for a host for my files I zipped them up but the file was still way to big.