What you're doing seems to be right: Attach an AkGeometry node as a child to a MeshInstance3D.

While not directly answering your question, here are a few things that tripped me up at first:

• Try generating soundbanks and re-generating Wwise IDs

• You might also have to tell Godot to automatically load the Init.bnk. Go to Project Settings, toggle "Advanced Settings", scroll down to the bottom and under the Wwise settings, see "Common User Settings" and check "Load Init Bank at Startup"