I've been using Facetime on my imac 10.6.8 for about a year. Generally speaking, it works quite well. But, once in a while we experience the same problem as you do. We communicate regularly with ipads and iphones.
Sporadically, and all of a sudden, they can't hear us. I'm thinking that it may be a glitch in the Facetime for Mac software, as this complaint is regularly made on this forum. For my wife and I it's a problem we can live with because when it does happen we can re-engage our contacts with full sound, both ways, if we end the call and restart it. I do notice that the chances of it occurring are greater when my wife starts trying to scream over the background noise that our grandkids sometimes make. LOL