firebase cloud messaging does not play sound

I have my backend in php sending firebase cloud message and I am receiving the message to my frontend without a problem but for some reason I’m not getting the custom sound.

this is the request:

$url = "https://fcm.googleapis.com/fcm/send";

$notification = array(
'title' => $title,
'body' => $body,
'click_action' => 'https://ww.google.com/',
'icon' => 'default',
'sound' => 'path-to-sound',

);

$arrayToSend = array(
'to' => $token,
'notification' => $notification,
'priority' => 'high'
);
$json = json_encode($arrayToSend);
$headers = array(
'Content-Type: application/json',
'Authorization: key=' . $serverKey
);

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, $url);
curl_setopt($ch, CURLOPT_CUSTOMREQUEST, "POST");
curl_setopt($ch, CURLOPT_POSTFIELDS, $json);
curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$response = curl_exec($ch);
if ($response === FALSE) {
echo 'FCM Send Error: ' . curl_error($ch);
return 0;
}
curl_close($ch);

And this is the payload in the frontend:

{
"from": "243712765598",
"priority": "high",
"notification": {
"title": "title test",
"body": "body test",
"icon": "default",
"click_action": "https://www.google.com/"
},
"fcmMessageId": "2e674039-9920-4bcb-9f6b-da9e6e103cb9"
}

I am not getting the sound in my object (I’ve made sure the path to the sound is correct).