There are 3 known versions of the Sonic CD Beta to exist. They are given version numbers based upon the dates they were revealed to press or as demos.
Version 510 10/5/93
This is the earliest version and contains 3 incomplete levels. The title screen is distinctly different from the final version as is the music.
Version 712 12/7/93
This is a relevantly complete version. There are only a few differences between this and the final version except for the incomplete later levels.
Version 920 20/9/93
This is a complete version of the US game yet with the original Japanese soundtrack. The game though is identical to the final version. This should be considered a pre-release.
You can download the Sonic CD Beta versions from this website.