I'm going to assume the CFM is actual cubic feet per minute and not at standard conditions.
If the 680 cfm refers to standard conditions, you'll need to convert to actual conditions.
ACFM = 680 SCFM * (14.7/P1)*(T1 + 460)/520
ACFM is actual cubic feet per minute
SCFM is standard cubic feet per minute
P1 is flowing pressure, psia (psig + atmospheric pressure)
T1 is flowing temperature, deg F
If your flow rate is at standard conditions, you can neglect the 5" WC, its effect is neglible.
Once you know the actual flow rate and the duct size, it's simple to calculate the average velocity. All you need to do is divide the actual flow rate by the area of the pipe/duct it is in.
For example, 4" sch 40 pipe has an ID of 4.026 inches or an internal area of 0.0884 ft2. Therefore, the velocity is 7690 ft/min.
