No "session" in this Request Type. Check: https://developer.amazon.com/es-ES/docs/alexa/custom-skills/request-and-response-json-reference.html#request-format

AudioPlayerRequest-System.json

{
    "version": "1.0",
    "context": {
        "AudioPlayer": {
            "offsetInMilliseconds": 1664,
            "token": "4",
            "playerActivity": "PLAYING"
        },
        "System": {
            "application": {
                "applicationId": "amzn1.ask.skill.e08b758f-07cd-4995-bba4-c52b9abd212b"
            },
            "user": {
                "userId": "amzn1.ask.account.AGBY4Z6GMAWZ3PUYRY4PWYZP73O2HU6EEMV5JOX7YPL7WT6USSGTLFNALBRVWSDITXSDFZA5CYA64O23VL5SCEQTVLGHODMX2QCOCNUWP4PSBOT5US3V4MJIFZEVUOEHRWADX7VK2OR3DK74UFQLMFVQSELPAF53K53B2SG32ZKSZORWUOL5JLGVHBB66MVJEXN5OVMCES3CPYY"
            },
            "device": {
                "deviceId": "amzn1.ask.device.AFRPJJGBRWZTXOJOLDRQGDSB2I6GKEC7YHKU5COXQMGZS2QMQIQTVCPGQHPMNWMMVPPZWSUK77MJ5PSKZRP7PZE2TTASBDCHOHV6EQQ6KDARY4GGOZH4TX5M5QG5JWT3G2DXYSACOP4V3IKAKCLEGVVR625NI3AHJAAUHOULQULJGALGVSIVW",
                "supportedInterfaces": {
                    "AudioPlayer": {}
                }
            },
            "apiEndpoint": "https://api.eu.amazonalexa.com",
            "apiAccessToken": "eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiIsImtpZCI6IjEifQ.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.INSdP_spG1Vo0ZsclFqvgXQFIS9dt64wvCTojOAtcWrsDf5HO4_RGu_1xWStx2ySDxhVI_uyCWUJgiYdMYwQad51lI27hmQL8y67siNxBVzQ5uAdIatauaLpaa8n7QJlIKh2pHOJkthXDh2o48fGjflue-YJt8B85d77yvX7AA-35C1rNDoMKtHpXFFtqLZ44To2NL9kOFDCAqlWWqrjJmrIKr8yxGicu4aOLRB8BrkbOrhIcUkn8PEC8NHj9cDwNL7vL23i2_0eIAiHNraBgfYMC8z1DULihPeI4fZO3dsvPrOwkiT5LnlZ-m0wFkqZsQFUt5BiKOPEUzFeW0OX5g"
        },
        "Viewport": {
            "experiences": [
                {
                    "arcMinuteWidth": 346,
                    "arcMinuteHeight": 216,
                    "canRotate": false,
                    "canResize": false
                }
            ],
            "mode": "HUB",
            "shape": "RECTANGLE",
            "pixelWidth": 1280,
            "pixelHeight": 800,
            "dpi": 160,
            "currentPixelWidth": 1280,
            "currentPixelHeight": 800,
            "touch": [
                "SINGLE"
            ],
            "keyboard": [],
            "video": {
                "codecs": [
                    "H_264_42",
                    "H_264_41"
                ]
            }
        },
        "Viewports": [
            {
                "type": "APL",
                "id": "main",
                "shape": "RECTANGLE",
                "dpi": 160,
                "presentationType": "STANDARD",
                "canRotate": false,
                "configuration": {
                    "current": {
                        "mode": "HUB",
                        "video": {
                            "codecs": [
                                "H_264_42",
                                "H_264_41"
                            ]
                        },
                        "size": {
                            "type": "DISCRETE",
                            "pixelWidth": 1280,
                            "pixelHeight": 800
                        }
                    }
                }
            }
        ]
    },
    "request": {
        "type": "AudioPlayer.PlaybackStarted",
        "requestId": "amzn1.echo-api.request.60ad09e9-ce73-423a-ac8e-2d89cd1999cb",
        "timestamp": "2020-04-11T11:38:39Z",
        "locale": "es-ES",
        "token": "4",
        "offsetInMilliseconds": 1664
    }
}