Adaptive Card embedded audio doesn't play

Hi Everyone

I have an adaptive card that was working until this week. I've got a workflow that sends an email when a voicemail is received, then saves the voicemail in SharePoint and sends the audio file embedded in an adaptive card in Teams. The code isn't complicated, I've just put the sharing link as a variable and then it's more or less like this:

"type": "Media",
"sources": [
{
"mimeType": "audio/wav",
"url": "@{variables('VoicemailLink')})"
}
]

It was working last week, and I didn't change anything. I've put a quick workaround in to put a hyperlink to the voicemail in the text of the adaptive card, and that seems fine so the variable link is working and the saved audio file is working, it's just not embedding the file nicely into the card. It opens up in a browser window instead.

Has there been an update to adaptive cards or Teams that might break this sort of thing? 

Thanks for your help.