Unfortunately, I strongly suspect that you will not be able to do what you want because I'm not sure it is even possible to download such an "old" version. And the fact you do not have your serial number will surely prevent you installing and running it if you are able to obtain a download.
Perhaps you should consider buying the latest version?
OTOH I sincerely hope you are not posting here in the hopes of obtaining a cracked copy of the software! 😇
Hi - I do have a registered version of Sound Forge 9.0 which works perfectly for my needs. However, I've switched computers and cannot find the disk and there is no download. I don't want to spend another $300 on a new version. What can I do?
As in my reply to the OP I rather doubt that anyone can actually assist you with this. The only suggestion I can make is that you contact Magix Customer Service here, selecting the "Purchase consultation contact" option, or use the email option of email@example.com.