Hi.
It's already have been discussed in here.

.shoutbox {
    display: none;
}

.page-home .shoutbox {
    display: block;
}

Obviously there is dependencies issues, but this is as far as I can help you.
Maybe @julian or @psychobunny can help.

@yariplus Thanks. Works like a charm.

@psychobunny I tried your method and it's working.
Except this little mistake

.page-recent .my-widget {
  display: none;
}

Is there a way to do the opposite?
Show a widget on only one page?

On the last lines, you get a Address Already in Use Error.
This means you didn't stop your forum before starting it in dev mode.

Beside that, I don't see any issue.

Sorry, but im out of ideas here.
I'd suggest to launch your nodebb in dev mod ./nodebb dev
and see if anything strange appears.
Try enabling & disabling soundcloud plugin and post a soundcloud link.

I don't have the answer, but since it's working on your first site, it's not plugin related.
It is a configuration issue.

My last guess would be to check in markdown plugin options. See if Autoconvert url-like texts to links is checked.

Just a guess, but maybe it's your plugin ordering.
Check in the plugin page, and see in "order active plugin"
You should order soundcloud plugin after the markdown plugin.
Hope it helps.

@MJ Do you still have issue with this.
I updated the plugin to appear on 0.9.0 plugin list.
But seems to run fine.

Fixed on master branch