The Slew Ext Volume block for the ADAU1452 allows envelope shaping like nothing previously available. At first I could not get a fast attack and slow decay from it, but here's the trick: The "up" and "down" controls are reversed in relation to their effects. Adjust the second control for attack and the first for decay.
The settings shown provides a fast attack and a slightly lingering decay, both via ear and on the Real-Time Display.
RE: Using the ADAU145x's Slew Ext Volume Block by KJBob:
This and another issue with the -1452 External Slew Volume block have been fixed in SigmaStudio Beta 11.1.