In asciidoctor-reveal.js a simple video block like
video::./my-vid.mp4[options="autoplay,muted"]
does autoplay, but is not muted — indeed, muted is nowhere to be found in the resulting HTML.
This issue appears to be fixed in Asciidoctor proper (see asciidoctor/asciidoctor#3408); maybe that just needs to make its way into asciidoctor-reveal.js?