In macOS, notification sounds can be adjusted in macOS > System Preferences > Notifications > Postbox > Play sound for notifications.
For both macOS and Windows, you can change the sound Postbox makes when a new message arrives:
- Open the General preferences window.
- (macOS) Postbox > Preferences > General
- (Windows) Tools > Options > General
- Click Browse to select the sound file to use. The file must be in .wav format.
- Select Play the following sound file.
- After you have selected the file, click Open.
To test the sound file after you've set it, click Play.