Codec2 might be patent-free, but Codec2 with a WaveNet decoder isn't because WaveNet (convolutional neural networks for generating audio sequence data) is patented: https://patents.justia.com/patent/20180075343
Welcome to software patents.
Question for IP experts: now that I have heard of the existence of WaveNet and a rough idea of how it works (training a neural network to decode low-bitrate speech data with as much fidelity as possible to the original), would I be prohibited from selling a similar product built with the same technique? How about if I had never heard of WaveNet and went about doing the same thing?
BUT: patents are far more specific than just "a neural network to decode low-bitrate speech data with as much fidelity as possible to the original)". Starting with that goal, you are unlikely to recreate WaveNet's specific structure that is patented.
In fact, WaveNet describes a more general method to efficiently work with sound signals, somewhat comparable to convolutions for images. It's also not impossible to work with sound using alternative MM structures that are not patented, and might actually perform better than WaveNet.