That is because your game isn't linking against the Wwise SDK libraries; the AkAudio module (found in the engine) is.
We have wrapped all Wwise SDK functionality in the FAkAudioDevice. Should you want to add some functionality to it, feel free to do so.
For example, you could add GetIDFromString to the FAkAudioDevice (found in UE4\Engine\Source\Runtime\AkAudio\Private\AkAudioDevice.cpp), and then, from your game code, add this:
#include "AkAudioDevice.h"
[...]
FAkAudioDevice* AudioDevice = FAkAudioDevice::Get();
if (AudioDevice)
{
uint32 WwiseID = AudioDevice->GetIDFromString("MyString");
}
In other words, the FAkAudioDevice is the UE4 wrapper to the AK::SoundEngine workspace. Use this within your game.