Macbook Pro 2018: No Sound over Displayport after sleep

i use a Macbook Pro 2018 13" with the last Mojave Version. For better work, i use the LG 34WK95U-W as external Monitor over Thunderbolt 3. Always is fine. The Macbook is still loading over the Monitor and i have a great big Display. But i can't use the Sound-Speakers from the LG-Monitor. When i restart the MacBook and i am logged in, i can use the speakers. After the Macbook was sleeping, no sound over the LG Speakers. When i restart the Macbook again, it works.

Where is the issue and how can i fix that?

Thanks for your help...

Have you updated macOS to 10.14.4? It supposedly addressed some issues with LG monitors. And one question: how long does it take the monitor to wake up from sleep?