Deep-learning-based compression techniques may one day be able to get speech down to several hundred bits per second, and non-speech audio in not much more. (They share the computational expensiveness problem though; even more so.) Google's Lyra seems to perform similarly to Opus for speech, at less than half the bitrate: https://ai.googleblog.com/2021/02/lyra-new-very-low-bitrate-...