{
"to": "XXXX",
"notification": {
"title": "ASAP Alert",
"body": "Please open your app"
},
"data": {
"screen": "/Nexpage1",
"sound": "alarm",
"click_action": "FLUTTER_NOTIFICATION_CLICK"
}
}
Above is my payload for the push notification. I have insert the alarm.mp3 file inside the raw folder, however it still does not give me the alarm sound, i have try for alarm.mp3 also, is there anything wrong with the json? of it because of the code on my dart file?

Playing Custom Sound for a Local Notification Message To play a custom sound for a local notification message, you should add your sound file as a raw resource. The path for raw resources is <your_flutter_project>/android/app/src/main/res/raw/. Check the Flutter Sample Code of the plugin for a working demonstration.
On the Notification category page, scroll down to the Sound section. It shows the default tone enabled for the app. Tap Sound and select your desired notification tone from the list to change the presets.
In the main app target, select Signing & Capabilities > All > + Capability and then search "push." Double-click on Push Notifications to enable it. Next, enable Background Modes and check Remote Notifications. Now you are ready to send notifications from the OneSignal dashboard!
Reading this it seems that it should be manage automatically (if you didn't use a notification builder) on Android but you have to specify the .mp3 extension too and put it inside notification field and not data one..
"sound": "alarm.mp3"
iOS behaves very differently under the hood but you can use a custom sound by setting the sound: field in the notification payload too. Anyway .mp3 is not a valid APN notification file format, and you need to specify also the file extention.
"sound": "filename.caf"
Follow Apple documentation in order to forge your custom sound file for your app.
mp3 is not a valid format
Preparing Custom Alert Sounds
Local and remote notifications can specify custom alert sounds to be played when the notification is delivered. You can package the audio data in an aiff, wav, or caf file. Because they are played by the system-sound facility, custom sounds must be in one of the following audio data formats:
Linear PCM
MA4 (IMA/ADPCM)
µLaw
aLawPlace custom sound files in your
app bundleor in theLibrary/Soundsfolder of your app’s container directory. Custom sounds must be under 30 seconds when played. If a custom sound is over that limit, the default system sound is played instead.You can use the
afconverttool to convert sounds. For example, to convert the 16-bit linear PCM system soundSubmarine.aifftoIMA4audio in aCAF file, use the following command in the Terminal app:afconvert /System/Library/Sounds/Submarine.aiff ~/Desktop/sub.caf -d ima4 -f caff -v
For exampole to convert your mp3 file in a caf file you could type in terminal:
afconvert -f caff -d LEI16 alarm.mp3 alarm.caf
Read this doc in order to have a deep inside of all generic and specific notifciation payload fields.
UPDATE
I've tested the Android part and I can confirm that putting your .mp3 file in res/raw/ folder the sound is played as documented and expected.
That's my notification payload:
{
"to" : "my_device_token",
"collapse_key" : "type_a",
"priority" : "high",
"notification" : {
"body" : "Test Notification body for custom sound {{datestamp}}",
"title": "Custom sound alert.mp3",
"sound": "alert.mp3"
}
}

I've tested also the iOS version after converting .mp3 file to .caf file in that way:
afconvert -f caff -d LEI16 alert.mp3 alert.caf
the same json payload with the different filename works:
{
"to" : "my_device_token",
"collapse_key" : "type_a",
"priority" : "high",
"notification" : {
"body" : "Test Notification body for custom sound {{datestamp}}",
"title": "Custom sound alert.mp3",
"sound": "alert.caf"
}
}
Remember to add the file in your main bundle.

That works if the app is terminated or in background.
If you want to show an alert and play a sound when the app is in foreground you have to manage it on onMessage event like someone already have told you here, or you can use a platform-channel here to build your own notification with a Notification.Builder on Android and a UNNotificationCenter on iOS (for example).
UPDATE
This issue has been solved. See here the official comment:
Hey all 👋
As part of our roadmap (#2582) we've just shipped a complete rework of the firebase_messaging plugin that aims to solve this and many other issues.
If you can, please try out the dev release (see the migration guide for upgrading and for changes) and if you have any feedback then join in the discussion here.
Given the scope of the rework I'm going to go ahead and close this issue in favor of trying out the latest plugin.
Thanks everyone 🤓
ShadowSheep did a good job at answering this question, but there's one thing I want to clarify for trying to get the iOS sounds to work.
You have to add the sound into XCode (which is where ShadowSheep speaks of including the asset inside of the main bundle). You can just drag and drop the audio file (in .caf or other supported format mentioned above) into the root directory (usually called Runner for Flutter) in XCode:

If you have done this and follow the setup described in the above question/answer, you should be in business.
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