First, we need you to clarify some things: Magix is the company who manufacture the software Sound Forge.
Please tell us exactly which Magix product you have bought and from where. We can assume it is Sound Forge but the actual place of purchase is very important.
Also, the message of an invalid serial number would suggest you are trying to use one that is applicable to something like Sound Forge Audio Studio but have actually installed Sound Forge Pro, or vice versa.
@Paul-Godwin Please check the same e-mail for additional serial numbers. Every software has its own serial. By the way, is Sound Forge Audio Studio 17 even included in Music Maker? I don't know.
I need to register Sound Forge Audio Studio 9.0d. I keep getting an error message saying my serial number is invalid. How do I find the serial number? It isn't printed on the CD.
You are asking about a different version of Sound Forge Audio Studio to the one listed by the original questioner so should really have been placed in a new thread.
However, since you are asking about a version produced before Magix owned the product line please read this post in the Magix Vegas forums and follow its advice.
If that is not the solution for you, you should log into your Magix account (if you have never set this up I understand your Sony account credentials will allow you to log into your Magix account), go to My products and check the details there.