We've been using Mumble for bigger conferences for almost two years now because we couldn't find a better alternative.

We tried Skype which is almost completely broken (used to work fine).

We tried multiple WebRTC solutions like appear.in which only work fine up to three people.

We tried Google hangouts, which worked fine but we didn't like the quality.

Mumble worked fine, but only if people use push-to-talk. Otherwise you get horrible noise. Every scratching goes over the line.

Skype Business supposedly works very well. We don't use it because we don't support Microsoft breaking the regular Skype.

We currently use the audio conferencing feature of (paid) slack. It works extremely well and has great noise reduction.