Mute iface says:
It should always be possible to mute an entire call. It is sometimes also possible to mute individual Contents (e.g. to prevent background noise from disturbing other participants, but remain visible on webcam) or to mute individual streams (e.g. to "whisper" to other call participants)
I understand this as Type.Call1 MUST implement Mute, Call1.Content and Call1.Stream COULD implement Mute.
Atm Call1.Content's spec says it MUST implement Mute, and Call1.Stream does not mention Mute at all.
In spec, we can use <tp:requires interface=""/> to make the extra iface mandatory, but we have nothing to tell it is optional. So I suggest just removing the tp:requires from Call1.Content, and add in both Call1.Content and Call1.Stream a note in its description that it could implement Mute.
Here is a branch clarifying this.
++ on the patch