As of the latest version it is not possible to assign custom notification tones. Previously the app recognized all the sounds stored in ~/Library/Sounds and it was possible to use them for notifications. Now it doesn't work anymore.
It would be nice if we could assign some custom notification sounds for the app because the default ones are kinda lacking.
Once again, previously it was working correctly.
Telegram macOS 8.1 (221723) STABLE, MacBookAir10,1, 11.4