what the frick...Dont keep sounds at 0, use scrollbar and increase the volume. I dont have this issue and I failed to replicate it (and to adjust code accordingly), but I know that for some having sounds muted prevent playing webm files.
Ren'Py by default plays WebM animations using the sound channel. If the player sets the sound channel volume to 0, Ren'Py can interpret this as muting not just audio, but the entire playback of WebM animations.
If there is any tech renpy guru here to advice how to keep optionality to increase and decrease sounds for animations... and at the same time avoid this issue, let me know.
Muting sounds "mutes" the animation. whoever coded RenPy like that