Since this is the first hit on Google for "Wwise ShortID", I'll use this to share some important details that aren't mentioned in the SDK.
The input for the FNV function should be the GUID bytes in mixed-endian order. The first three components are little-endian, and the last two are big-endian. In other words, convert the hex pairs in the GUID to an unsigned byte array and sort them this order: 3, 2, 1, 0, 5, 4, 7, 6, 8, 9, 10, 11, 12, 13, 14, 15.
Here's a C# implementation that returns the correct ID for both GUIDs and strings. If anyone else is stuck on this, feel free to use it as-is, or as a reference if you need it in another language.