One can play around with Opus by exporting audio streams from existing video and audio

    ffmpeg -i sample.avi -map a out.wav

and then encoding that wav to opus

    opusenc --downmix-mono --bitrate 32 --vbr out.wav cor.opus

to decode

    opusdec cor.opus final.wav

and then

    play final.wav

brew install opus-tools sox ffmpeg

In listening tests Opus typically outperforms HE-AAC by a fair margin. This is for 64 Kbit/s and under.