core/homeassistant/components/soundtouch/icons.json

{
  "services": {
    "play_everywhere": {
      "service": "mdi:play"
    },
    "create_zone": {
      "service": "mdi:plus"
    },
    "add_zone_slave": {
      "service": "mdi:plus"
    },
    "remove_zone_slave": {
      "service": "mdi:minus"
    }
  }
}