You can check out the Early Access version that has shipped but also follow along with development in UE5/Main on github. https://github.com/EpicGames/UnrealEngine/tree/ue5-main
MetaSounds are currently entirely written in a plugin and they can be extended by other plugins. Here's the MetaSound plugin source -- https://github.com/EpicGames/UnrealEngine/tree/ue5-main/Engine/Plugins/Runtime/Metasound
Probably the easiest way to experiment and get started is to just check out any of the existing MetaSound nodes and use them as a crib for how to make a new node. It's as easy as dropping in a new (single) cpp file in the project and compiling it. Check out specifically this folder: https://github.com/EpicGames/UnrealEngine/tree/ue5-main/Engine/Plugins/Runtime/Metasound/Source/MetasoundStandardNodes/Private
Here's, for example, the source for the stereo delay node: https://github.com/EpicGames/UnrealEngine/blob/ue5-main/Engine/Plugins/Runtime/Metasound/Source/MetasoundStandardNodes/Private/MetasoundStereoDelayNode.cpp
Note the boilerplate that is needed to define inputs/outputs, tooltips, vertex (pin) registration, types, defaults, etc. There is some boilerplate! One interesting technical detail of MetaSounds is that they feature "sample accurate" triggering for events through a special trigger pin type. Here's a node that shows that working:
https://github.com/EpicGames/UnrealEngine/blob/ue5-main/Engine/Plugins/Runtime/Metasound/Source/MetasoundStandardNodes/Private/MetasoundTriggerCounterNode.cpp Note the bifurcation of the render block using lambda's on the trigger reference
https://docs.unrealengine.com/5.0/en-US/AudioFeatures/