I don't think that is the problem, because a voice message received prior to this one works fine and so do the others, this is a first timeHave you by any chance switched on the Mute switch on the side of the phone ?
I will talk to my friend and c if the problem is at his endThere may have been a recording problem at the other end. I have also had messages where there is no sound because the caller has accidentally activated the phone. This can happen with iMessages as well, and an empty bubble shows instead of a message.