I just wanted to post this solution in case anyone else might be having the same problem I had. 

Problem:  When using Sonar 6 PE in Windows 7, the sound would randomly stop working - not just within Sonar, but in Windows entirely.  Rebooting the fireface returns sound, but it loses it again after soon enough. 

The solution that worked for me was to go into Windows Device Mangler and switch to the Legacy firewire driver, as explained here. 

Fireface 400 (driver v. 3.059, firmware v. 1.70)
Windows 7 Home Premium 64 bit
Asus P8P67 Pro
Intel i5 2500k