From freedv.org:

FreeDV is a suite of digital voice modes for HF radio. Our flagship mode is the Radio Autoencoder (RADE). You can run RADE using a free GUI application for Windows, Linux and macOS that allows any SSB radio to be used for high quality digital voice.

I haven't tried it yet, but it looks pretty interesting. And I really like that it's Free/Open Source and cross platform.