Osmo-GMR now implements voice codec decoder
GMR-1 uses an undocumented codec in the AMBE family and as such Osmo-GMR didn't have any implementation of it and converting TCH3 frames to actual audio wasn't possible.
Now, after quite a bit of effort to figure things out, I'm pleased to announce that an open-source decoder is available. It's not perfect and doesn't match the quality of the official decoder but it does produce intelligible voice and will have to do for now.
It's currently available in a separate 'sylvain/codec' branch in the git but should be merged into master soon after a last review of the code.