c:a=copy,hls_time=4,hls_playlist_type=vod,hls_segment_type=fmp4,hls_list_size=0,hls_segment_filename='audio-%d.m4s'
Those aren't codec options. The `c:a=copy` is a convenience option and the rest are for the `hls` muxer. Pretty easy to distinguish what's an encoder option and what's not: the ones you replaced in the x264 example can be found in https://ffmpeg.org/ffmpeg-codecs.html; yours are in https://ffmpeg.org/ffmpeg-formats.html. You can't just cram whatever options you specify on an FFmpeg command line, libav* don't work like that. The FFmpeg frontend is mostly outside the libav* libraries.
Study the `format` part of the API, e.g. [`format::open_with`](https://docs.rs/ffmpeg-next/4.3.8/ffmpeg_next/format/fn.open_with.html), to use muxers.