Application Hanging Diagnosis

Unfortunately, to the best of my knowledge Windows usually has no way of knowing the cause of a hang. All it knows is that an application has taken too long (> 5 seconds) to dispatch a Windows message. It can identify the application, but that's all. What exactly caused the application to get stuck is known only to the application.
 
In fact, Windows doesn't even know if the app is really hung or just taking a long time to perform some operation. A "true" hang is when the software is waiting for something to happen that isn't going to ever happen, such as an infinite loop. More often, it's waiting for something to happen outside the program such as a response from a device that's broken, shut down or disabled.
 
"Hang signatures" are a relatively recent thing, new (I think) at Windows 7. I have not found a good explanation as to what they really are, other than a way to indicate whether the hang you're getting today is exactly the same as the one you got yesterday. I don't think there is any useful information in them beyond that.
 
As for troubleshooting, it's a hit-and-miss process, starting with the most likely suspects: hardware and hardware drivers. I think you're right to place the audio interface at the top of the suspect list, although it could also be the USB port it connects through. Hopefully, the hangs are frequent. If so, you could try disconnecting the Onyx and enabling your motherboard's integrated audio while playing back a bunch of projects.