I had a need for an audible alarm separate from the NVR base unit so I wrote some lightweight micropython code to support the usecase.
For anyone here interested the code and hardware build information can be found here

https://[censored]/h3b4r1/reolink_relay

@user_597493679579258_597493679579258
The answer was to use GetMdAlarm

I'm currently working on a python script to pull alarm status in order to activate an external device (light, siren etc), I have the API working to the degree that I can pull a token and use it to integrate the NVR for status eg GetChannelStatus is working correctly. The issue I have is that GetAlarm is returning an error. I am using python requests with:

def alm_state(config,token):
    payload = [
        {
            "cmd":"GetAlarm",
            "action":1,
            "param":{
                "Alarm":{
                    "type":"md",
                    "channel":0,
                }
            }
        }
    ]
    return requests.post(f'http://{config["reolink"]["nvr_ip"]}/api.cgi?cmd=GetAlarm&token={token}', json=payload).json()

Token and URL are working, the json conforms to the latest API documentation and mirrors the Pypi request for the same information.

The response is as follows:

[
  {
    "cmd":"Unknown",
    "code"=1,
    "error": {
      "details": "not support",
      "rspCode": -9
    }
  }
]

The System is an RLN16-410 with 11 x RLC810A cameras attached.