I rebooted an RPi5 running piCore64 17.x with only openssh loaded onboot.
Loading vlc and alsa or mpv and alsa from the console prompt, I could play a test mp4 video with sound on an hdmi monitor with powered speakers connected to the hdmi monitor's sound jack.
Note that from the console prompt, the video resolution with both mpv and vlc is incorrect. If played from a gui, the video resolution is correct.
Note also that both mpv and vlc have been updated recently.