I've been using Google Hangouts. It works very well. Unfortunately it currently requires a binary plugin from Google to work with Firefox. Google provides a PPA so that I can get that plugin and updates to it through "apt-get". Apparently the next version of Firefox, or possibly the next-next, will include sufficient "websockets" support that the binary plugin will no longer be required.
In short: if you want something that works right now, use Google Hangouts. Creating something equivalently effective, but with complete freedom, security, and decentralization is certainly a worthy task that I hope someone will take on.
Oh, and I think you *always* need to use a USB headset for voip. Figuring out and cancelling the interference between speakers and mic is perhaps not a job that you should try to solve in software, if you can instead solve it by buying a USB headset for as little as $20.