I have written a guide how to “hack” notifications’ sounds HERE. For full reference, please read that article before continuing, if you haven’t already do so. “Thanks” to macOS Catalina limitations, you can no longer apply guide called “Cause 2: system sounds:”. It’s because macOS Catalina creates two partitions, one for system data and one for user data. System data partition is read-only to improve security of operating system, but it completely disables my hack from my guide.
I used this hack to change notification sounds for Telegram and Fantastical 2. Telegram only offers few notification sounds from macOS’s system sounds and I wanted the same sound Telegram uses on iPhone and iPad, so I downloaded sound file from Internet, converted to AIFF format and renamed it to “Frog.aiff” and with this sound file I replaced original “Frog.aiff” sound file in macOS’s system library. Then in Telegram app, I have chosen from offered sounds that “Frog.aiff” and done. I got the same tone on both Mac and iPhone/iPad.
As for Fantastical 2, on macOS it uses system notification sound called “Basso.aiff”. I asked Fantastical’s support for sound file from iOS. When I got it, I converted and renamed it to “Basso.aiff” and replaced original “Basso.aiff” in macOS’s system library. Then, I had the same notification sound for Mac, iPhone and iPad.
With macOS Catalina, things are changing. You can no longer use this hack. As for Telegram, I discovered that my first hack “Cause 1: applications’ sounds:” now works for Telegram. So it was simple, just copy your sound file to “~/Library/Sounds” and choose it in Telegram app.
As for Fantastical 2, this way only works partially. I copied sound file to “~/Library/Sounds” – same place as used for Telegram. Then in Fantastical’s 2 settings I have chosen my sound file for all types of alerts. When you create an event on Mac, you can enjoy your notification sound. But if you create an event on iPhone/iPad version of Fantastical 2, Mac version of Fantastical 2 will still use “Basso.aiff” from macOS’s system library.
I have found no way how to change it. Probably the only way is to ask Fantastical’s 2 developers to integrate notification sound directly to the app, as is done in its iOS/iPadOS counterparts.