What's making this sound on my mac?
That's the "new email" sound for Spark, Readdle's Mac mail client. A rather unique characteristic of Spark is that it will continue to send you notifications—audio and all—even when the app is not open. Even if you haven't seen any visible notifications, it's entirely possible that you've configured your Notifications preferences to hide them, but still play the sounds (check System Preferences > Notifications > Spark)—an example of such a configuration is shown below:



As Spark is exclusively a Mac App Store app, you can easily uninstall it from Launchpad (open Launchpad, click and hold on Spark, then click the x button that appears on Spark's icon). Once the app is uninstalled, it can no longer send you notifications.