For those interested in low bandwidth audio codecs, take a look at the voice codec used for Iridium handheld satellite phones, which was finalized in about 1998. Fully twenty plus years ago.
It doesn't sound the best, but consider the processing power constraints it was designed with...
Iridium appears to be using a vocoder called AMBE. Its quality is similar to the one of the MELP codec from the demo and it also runs at 2.4 kb/s. LPCNet at 1.6 is a significant improvement over that -- if you can afford the complexity of course (at least it'll work on a phone now).
Based on my previous experience with Iridium I believe it actually operates at a data rate up to about 3 to 3.2 kb/s. 2400 bps of it is actual usable voice payload, the remaining 600 bps is FEC.
Iridium data (not the new next-generation network) service is around the same speed, it's 2400 bps + whatever compression v42bis can gain you. For plain text and stuff it can be a bit faster, something that's already incompressible by v42bis will be right around 2400 baud.
It doesn't sound the best, but consider the processing power constraints it was designed with...
https://en.wikipedia.org/wiki/Iridium_Communications#Voice_a...