How to replace sounds in the plugin?

Plugin keeps sound notifications files in this folder:
/plugins/bp-better-messages/assets/sounds

You need copy this folder to another location in your website which will not be overwritten on plugin update.

For example inside the root folder of your website you can create folder sounds.

With this filter you can overwrite the folder in which plugin will search for sound files.

add_filter('bp_better_messages_sounds_assets', function($sounds_url){
     return 'https://yourwebsite.com/sounds/'; // New sounds url
});

Then you can replace sounds file for your own in that new folder.