Hear two beeps after wake
For me, it was calendar notifications. I noted the change when I upgraded to Mojave, that I got a notification sound whenever I opened the lid.  I had the calendar notification set to NONE - so there was no visual alert, but I had left "Play Sound" checked. It appears that if you have any notifications for the day, calendar alerts you on opening the lid. I toggled on the visual alert to confirm and, yep -- it was calendar alerts. Turned off sound and all was well. 

System Preferences > Notifications: