In 1999 I was the 2nd employee at what became one of the world's biggest music streaming services. I was working from home as we didn't have offices yet, and my broadband got cut off because I hadn't been paid yet and I was broke. I ended up having to re-encode the entire music catalog at 8000bps so I could stream it all over my 9600bps cell connection via the serial port of my Nokia 9000, when I was testing the system.
Opus is simply magic. Iirc, they now even have a 6kbps mode.
The thing is (and maybe this is a nitpick), once you are down to several hundred bps for speech, its getting to be more like speech-to-text (the encoder) and text-to-speech (decoder) than an audio codec.
I am actually not aware of any non-speech audio codecs which can go that low. Any links?
Spent about 4 months porting and then optimizing the wma decoder on a proprietary very-low-power dsp. It was hard work, but I got it down to about 12mhz for real-time decode on the dsp. The product this was intended for was going to be powered by a watch battery - hence the extreme optimization requirement. The source code, as received, used to consume ~400mhz on an athlon iirc. But with a lot of assistance from the dsp hardware and 4 months of elbow grease it could decode in less than 12mhz - for some profiles. Insane codebase!
I wonder if the decoder source is in the Windows source code leaks? I guess Windows Media Player source will be in there. That'd be worth a look for me to see how utterly horrible it actually was.