试解决成功。大致方法：若已安装mpv则先卸载，顺带卸载其余零星依赖组件，如gst-plugins-bad。之后在AUR库中找ffmpeg-git，用pamac命令行构建，在中途编辑配置文件中加入一行：
--enable-libsmbclient \
后开始编译安装，其版本为4.4。注意，rav1e可能需要同时编译安装，所需版本为0.4.0以上，从AUR库中找。两者同时构建可安装成功。
此时输入ffmpeg命令验证可见：
ffmpeg version N-100660-g71ce5c32f0 Copyright (c) 2000-2021 the FFmpeg developers
built with gcc 10.2.0 (GCC)
configuration: --prefix=/usr --disable-debug --disable-static --disable-stripping --enable-avisynth --enable-fontconfig --enable-gmp --enable-gnutls --enable-gpl --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libdav1d --enable-libdrm --enable-libfreetype --enable-libfribidi --enable-libgsm --enable-libiec61883 --enable-libjack --enable-libmfx --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-librav1e --enable-libsoxr --enable-libspeex --enable-libsmbclient --enable-libsrt --enable-libssh --enable-libtheora --enable-libv4l2 --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxcb --enable-libxml2 --enable-libxvid --enable-nvdec --enable-nvenc --enable-omx --enable-shared --enable-version3
libavutil 56. 63.101 / 56. 63.101
libavcodec 58.117.101 / 58.117.101
libavformat 58. 65.101 / 58. 65.101
libavdevice 58. 11.103 / 58. 11.103
libavfilter 7. 96.100 / 7. 96.100
libswscale 5. 8.100 / 5. 8.100
libswresample 3. 8.100 / 3. 8.100
libpostproc 55. 8.100 / 55. 8.100
Hyper fast Audio and Video encoder
usage: ffmpeg [options] [[infile options] -i infile]... {[outfile options] outfile}...
Use -h to get full help or, even better, run 'man ffmpeg'
注意其中--enable-libsmbclient已生效。此时再重装mpv，解决。