I have a MacBook Pro 15, with touchbar, connected via DisplayPort to a Lenovo LI2821 4k monitor . I also have an USB connection to the monitor. The USB for access to the integrated webcam, USB hard drive, speaker and mic.
When the MacBook Pro is first connected all of the devices work properly. After a short period of time, the speaker and mic stop working. For example, when viewing videos from YouTube, the video fails to start and I get the following message:“If playback doesn’t begin shortly, try restarting your device”. When I change the audio output from the USB audio to the DVI audio, the video starts immediately.
When I go into System Preferences, Sound and test the mic, there is no response. If I change that input to the Internal Microphone, that device works.