how to restore the "old" wavedata directory?

As I see is the global audio folder obviously only used for older projects. Recording all files of each project in a global folder was a mess.
When you open a new project you can manually choose an other folder for your audiofolder - just your wavedata disc.
Or you can save your projects on the same drive as your wavedata.
We use a SSD for projects and the wavedata inside the project folder.
Per projects folder will be easier to restore projects from a backup.
 

I would recommend you do what I (and others) do. And that is to have your Sonar projects directory on your audio drive and have all audio saved with the project. This does not impact performance, especially on modern systems. I think Sonar reads the project file into memory. So the only time it does anything with that file on disk is when you save.
 
The biggest advantage of having project audio saved with the project is that it is easier to manage and keeps everything together for safer backup and restore. I lost audio content years ago when I had projects and audio on different disks, because I transferred all my projects elsewhere and when I tried to use them all the audio was missing. Keeping it all together is so much better for loads of reasons and doesn't affect performance.