ffplay version 4.3.2 Copyright (c) 2003-2021 the FFmpeg developers
  built with gcc 8.4.0 (GCC)
  configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-amf --enable-avisynth --disable-cuda-llvm --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libaom --enable-libass --disable-libbluray --enable-libdav1d --enable-libdrm --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libjack --disable-libmfx --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --disable-libpulse --disable-librav1e --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libssh --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxml2 --enable-libxvid --disable-libzimg --disable-nvdec --disable-nvenc --enable-shared --enable-version3 --enable-lto --enable-libvmaf
  libavutil      56. 51.100 / 56. 51.100
  libavcodec     58. 91.100 / 58. 91.100
  libavformat    58. 45.100 / 58. 45.100
  libavdevice    58. 10.100 / 58. 10.100
  libavfilter     7. 85.100 /  7. 85.100
  libswscale      5.  7.100 /  5.  7.100
  libswresample   3.  7.100 /  3.  7.100
  libpostproc    55.  7.100 / 55.  7.100
Input #0, ogg, from 'file.opus':=    0B f=0/0   
  Duration: 00:00:07.79, start: -0.000167, bitrate: 109 kb/s
    Stream #0:0: Audio: opus, 48000 Hz, stereo, fltp
    Metadata:
      encoder         : Lavc57.89.100 libopus
      sample          : BDDRM.ST
                      : disthit.st
                      : FLTROPEN.ST
      extra info      : 1 pattern, 4 channels, 3/31 samples
ALSA lib pcm.c:2660:(snd_pcm_open_noupdate) Unknown PCM sndio
SDL_OpenAudio (2 channels, 48000 Hz): ALSA: Couldn't open audio device: No such file or directory
ALSA lib pcm.c:2660:(snd_pcm_open_noupdate) Unknown PCM sndio
SDL_OpenAudio (1 channels, 48000 Hz): ALSA: Couldn't open audio device: No such file or directory
ALSA lib pcm.c:2660:(snd_pcm_open_noupdate) Unknown PCM sndio
SDL_OpenAudio (2 channels, 44100 Hz): ALSA: Couldn't open audio device: No such file or directory
ALSA lib pcm.c:2660:(snd_pcm_open_noupdate) Unknown PCM sndio
SDL_OpenAudio (1 channels, 44100 Hz): ALSA: Couldn't open audio device: No such file or directory

When i use ffplay under sndio, this happens,  when I try iceweasel-uxp and go to invidio.us and play any given video, it doesn't work at all.

Also,  speakers and headphones are not available via alsamixer...

What I mean, is you only see master, pcm, and some weird other thing... I have no idea what the other thing is...
So yeah, theres a buggy issue, probably with the alsa-lib part of sndio... so it may actually be an alsa-lib issue, sadly...