Try opus. I was amazed how good that sounds at ridiculously low bitrates. I'd be surprised if you can tell the difference between 80kpbs opus and FLAC (i.e. raw) files outside certainly very rare corner cases. And unlike mp3, those corner cases don't sound terrible, though that's subjective. Even at 64kbps the difference wasn't obvious without careful listening - to me, YMMV :-).

If you do try it, make sure to use the 1.1 encoder (which deals with difficult samples by detecting that and upping the bitrate more aggressively than previous versions), and you might as well increase the maximum framesize the maximum (60ms) since you're not interested in low-latency applications.