If you use Google Voice, then you're familiar with getting notifications via email alerting you to a new voice mail message. The email contains a transcription of the message plus a link to go and listen to it.
Now, Google has updated the alert so that you can play the message directly in your Gmail acocunt. In order to access the new feature, you need to turn on the Google Voice Player in the Gmail Labs tab.
When you've listened to a voice mail in Gmail, it will sync with your Google Voice inbox. This means that once listened to, it will essentially "mark as read" in your Google Voice account as well.