The audio is out of sync with the video
Quick answer: Close the workout and reopen it. If it persists, check your connection, restart your phone and update MOMDAY. If it still happens after that, it is likely a bug, so please report it.
Work through these in order
End the workout and tap Resume.
Switch between WiFi and cellular.
Close MOMDAY fully and reopen it.
Restart your phone.
Update MOMDAY from the App Store or Google Play.
Using Bluetooth headphones? Disconnect and reconnect them.
Still out of sync?
That points to a bug. Report it in Help & Support in the app with your phone model, app version and which workout it happened on. A screen recording is a big help.