I think for most of these, the speech is more structured or we are talking to people that we may deem there's less pressure to be fluent to. When you whisper, sing or read out loud. You are reducing, speech is less spontaneous and more planned and structured. You don't need to think about words to say when reading or formulate them. Also you are reading out loud alone, so there's zero pressure to be fluent. Whispering and signing, is also structured speech because we have an established rhythm, pitch, per-determined /clearly defined words in the case of singing. Talking for talking in union, this actually involves what's called choral speech. In stutterers, research has found that there is irregular brain function in the wernikes area of the brain, the part responsible for auditory feedback. Hearing yourself talk also plays a role in speech function. If you lose your hearing or it's impeded in some way, your will find it very difficult to speak. In this area of the brain, research has found that there's less blood flow in this region when viewed on a MRI scan, so the wernikes area is under-activated when speaking. Talking in unison helps make up for that deficit in auditory feedback. DAF devices and smartphone apps (DAF Professional) make use of the same choral effect. However DAF have the opposite event in non-stutterers and cause disfluency in them.