Shoutbox

AFAIK, built in sounds are sent a different way to custom sounds, which is why you cannot do what you want.

For custom sounds, you use /sound <id> or /sound sound-name, so if you enter any text afterwards it will fail to find the sound.

The only way I can see a solution to your problem is to use quicktexts, by setting a shortcut you want which sends the sound you want, followed by the text you want (see images below for example)

Quicktext window

Chat Window

However, sometimes the sound will be sent AFTER the message.

Hope this helps.