This is because it's an section level ambox, and all small amboxes are stripped by MF

Even if the template is shown, the audio probably won't be usable due to T74975

In T94593#1167689, @TheDJ wrote:

Even if the template is shown, the audio probably won't be usable due to T74975

Won't be usable on mobile web.

Generally, audio clips work in the Android app.

Another note, this template uses the 'style classes' of mbox, not any of the semantic classes (ambox,cmbox,tmbox etc) which MobileFrontend should probably use for it's selection.

Proposed a change to template:sister https://en.wikipedia.org/w/index.php?title=Template_talk:Sister_project&action=history

Apparently the sister boxes are the reason that mbox-small is hidden completely. By adding a semantic class, we can narrow the scope of the MF hack.

Change 200934 had a related patch set uploaded (by TheDJ):
Limit hiding of sideboxes to sister sites

https://gerrit.wikimedia.org/r/200934

Change 200934 merged by jenkins-bot:
Limit hiding of sideboxes to sister sites

https://gerrit.wikimedia.org/r/200934

In T94593#1172301, @gerritbot wrote:

Change 200934 merged by jenkins-bot:
Limit hiding of sideboxes to sister sites

https://gerrit.wikimedia.org/r/200934

Is this rule (and its predecessor) thought for English Wikipedia only?