We use Mattermost in a small team and when it comes the time to chill out, we sometimes abuse of the #random channel
We loved the emoticon customization and we would love to see the possibility to send a sound to react to a message with the right movie quote for example.

That could be a plugin with a command like /play, I just doubt that the plugin would have the right to play a sound in browser.

The sound (and the command) could be created by users just like emoticons. The user should have to upload a sound file (like .mp3) and defines its label. Then, he could types /play my-custom-label-sound to trigger it.

Of course, a preference to disable those sound or only enable them when the user have the focus on the channel should be present.