You can fork the project and replace them and build, or do it locally by unpacking the app.asar and replace the ogg files.
(last suggestion not recommended, updates will overwrite the asar)
The ability to at least change the default sound for mail notifications is pretty common. So yes, I consider that a must have. Sounds based on email filters would be a way for Mailspring to stand out.
I’m new to Mailspring and just deleted the two other mail apps that I was testing. Adding a setting to change the notification sound and/or adjust the volume of the notification (relative to other system sounds) would be a great addition.
I’d like to vote for this feature too: A custom sound would be great! As I’m on windows it would be sufficient if Springmail would trigger the system sound for new mail.
Best regards - Ulrich