What's making this sound on my mac?

A few months ago, my laptop started playing a strange sound alert, but it's not from anything I recognise. There is no visual clue as to what it's related to. Just the sound plays, most commonly when I've just woken my laptop back up from having the lid closed, but also at other random times if I've been working on my laptop for a while. I'm worried it's some sort of monitoring or capturing app, but it perhaps wouldn't make sense for this type of program to have a sound alert. I just want to be able to stop it. I've gone into Activity Monitor, but can't seem to trace it from there. I've uploaded the sound file here if anybody recognises it: http://www.occupied.com.au/weirdsound.aif

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.