The stock air filter box on the SAAB 9-5 muffles most of the sound from the turbo. Replacing it with a universal high flow air filter lets loose the sound from the turbo. Any compact "cone" style filter with a 3 inch connector will fit and the difference in sound is dramatic.
A detailed "how-to" for the removal and replacement of the air filter.
Getting a performance diverted valve, like the one made by Forge, and installing it in the reverse configuration will further enhance the sound of the turbo.