You can get the empty attributes with boolean attributes in Slim. I guess your HTML snippet translates to something like this:
audio controls=true autoplay=true loop=true muted=true
source src="horse.ogg" type="audio/ogg"
source src="horse.mp3" type="audio/mpeg"
| Your browser does not support the audio tag.
or shorter
audio(controls autoplay loop muted)
…