<< For those of you who already use Google Voice, you're probably used to receiving voicemail notifications via email. A couple of minutes after someone leaves a voicemail on your Google Voice number, you'll receive an email showing who called, an automated transcript of the voicemail, and a link to play the message. You can click the link to listen to the message right from your computer.

Previously, clicking "Play message" opened a new page in your browser, but starting today, you can play voicemails right in Gmail. Just turn on the Google Voice player from the Gmail Labs tab under Settings and whenever you get a voicemail notification, the player will appear right below the message itself. >>

http://gmailblog.blogspot.com/2009/09/new-in-labs-play-google-voice-messages.html

Many of us uses a third party voicemail service where voicemails end
up in our inbox. Would be great to play wav files directly in Gmail, and not only mp3 files and Google Voice messages.