That library was last seen in TCL10 x86_64's libva.tcz. If I grab that extension from the repository and load it, then mpv seems to complain about something more serious:
bruno@x200:/mnt/data/Downloads$ wget http://repo.tinycorelinux.net/10.x/x86_64/tcz/libva.tcz
Connecting to repo.tinycorelinux.net (89.22.99.37:80)
saving to 'libva.tcz'
libva.tcz 100% |********************************| 86016 0:00:00 ETA
'libva.tcz' saved
bruno@x200:/mnt/data/Downloads$ tce-load -i ./libva.tcz
libva.tcz: OK
bruno@x200:/mnt/data/Downloads$ mpv
ffmpeg library versions:
libavutil 56.22.100 (runtime 56.70.100)
libavcodec 58.35.100 (runtime 58.134.100)
libavformat 58.20.100 (runtime 58.76.100)
libswscale 5.3.100 (runtime 5.9.100)
libavfilter 7.40.101 (runtime 7.110.100)
libswresample 3.3.100 (runtime 3.9.100)
ffmpeg version: 4.4
mpv was compiled against a different version of FFmpeg/Libav than the shared
library it is linked against. This is most likely a broken build and could
result in misbehavior and crashes.
mpv does not support this configuration and will not run - rebuild mpv instead.
Exiting... (Fatal error)
I think mpv probably needs to be recompiled. Juanito is the maintainer. I could volunteer to recompile it if it would be helpful.