Upgrade Firefox to version 61 and enable sndio
As Mozilla is deprecating ALSA support in Firefox, sndio is the best option for sound support. sndio is a lightweight audio framework developed for OpenBSD and a much better solution than PulseAudio.
Patches for building Firefox 61 with sndio support can be found in the Void Linux package repository:
https://github.com/void-linux/void-packages/tree/master/srcpkgs/firefox
Tips for making Firefox work with sndio can be found here:
https://github.com/void-linux/void-packages/issues/369
(from redmine: issue id 9041, created on 2018-06-28, closed on 2019-05-03)