LG 27UD88W No sound out of Headphone Jack

Hello and welcome to Apple Support Communities, yes2598!

I understand you're not able to hear audio through your monitor. I'd be happy to provide information to help you address this.

Give the following steps a try: If you can’t hear sound from your Mac speakers

"If you’re trying to listen through external speakers

• Make sure the speakers are plugged into the audio out port on your Mac or display (or the audio port , if your Mac has only one audio port), and if necessary, into a power supply. Check for a power switch and volume adjustment on your external speakers. If necessary, turn the speakers on and adjust the speaker volume.
• On your Mac, choose Apple menu  > System Preferences, click Sound, then click Output. If your Mac has only one audio port, click the “Use audio port for” pop-up menu, choose Sound Output, then make sure your external speakers are selected.
• If you’re trying to listen through speakers or headphones connected only to the USB port on your Mac or display, choose Apple menu > System Preferences, click Sound, then click Output. Make sure the external USB speakers are selected. Open System Information, click USB in the Hardware section of the sidebar, then make sure your speakers are listed. If they aren’t, try unplugging the speakers and then plugging them in again.
• If your external speakers still aren’t working, check the instructions that came with your speakers."

Have a great day!