matrix-spec/event-schemas/examples/m.room.message#m.audio

{
  "age": 146,
  "content": {
    "body": "Bee Gees - Stayin' Alive",
    "url": "mxc://localhost/ffed755USFFxlgbQYZGtryd",
    "info": {
      "duration": 2140786,
      "size": 1563685,
      "mimetype": "audio/mpeg"
    },
    "msgtype": "m.audio"
  },
  "event_id": "$143273582443PhrSn:localhost",
  "origin_server_ts": 1432735824653,
  "room_id": "!jEsUZKDJdhlrceRyVU:localhost",
  "type": "m.room.message",
  "sender": "@example:localhost"
}