There are some circumstances which require you to keep your MacBook silent (i.e.: mute your volume) in the event that a system sound is generated (such as a new message or email arriving). In a work setting, a meeting would be an obvious situation where these sounds can cause an unnecessary distraction to the discussion. The problem is, you still want to be notified when an event happens. There's always the badge notification on the dock (a visual indication)...
... but if your eyes aren't constantly fixed on the Dock, there's a good chance you will miss it. Here's a great solution for you.
OS X has a set of accessibility features, designed primarily for those with disabilities (hearing, seeing, etc). One of those features enables a short flash burst each time a system sound is generated (intended for those with hearing disabilities). Subtle enough that most people around you won't notice it, but powerful enough that it will catch your attention if you happen to be looking off into the distance.
To enable this feature, open System Preferences, then click on Accessibility. In the Audio section, check the "Flash the screen when an alert occurs" box. To test it, click on the "Test screen flash" button.