It turns out the issue is caused by the browser’s autoplay policy.
Google’s autoplay policy states:
Autoplay with sound is allowed if:
- User has interacted with the domain (click, tap, etc.).
This explains why toggling mute on twitch.tv fixed the muted flag in the embed url.
To fix the issue on Chrome, add the attribute allow=“autoplay” to the embed iframe. See their section on iframe delegation.