Jake visits his father's grave and you can see the freshly-turned dirt and a large headstone. Most cemeteries require a waiting period of several months for the dirt around the grave to settle before an upright, stone memorial or headstone can be added.
Devon is wearing headphones while playing back his recorded conversation with Jake, and his mother hears it through the door. If he's listening to the conversation through the headphones, she shouldn't be able to hear it. If he were listening to music through the headphones, then he wouldn't have been able to hear the conversation.