Been coming back to this forum to see if anybody comes up with a fix, but your post is worrisome as i cant see myself going for a replacement four (4!) times. It does feed the argument this being a software issue, though. Might be a combo with apps being used so here goes; i use my phone for mostly emailing, dont chat much, lots of planning agenda (with siri if it would work), making (voice) notes, some pictures, skype always open, imessage with voice messaging, i use some running tracker apps and a lot of safari but thats about it, really. The wierd thing is that in whatsapp for example the mic (front or back, dunno) is working fine. In apps like Notabilty, voice notes are fine. Its just apple apps siri, voice notes in imessage.
Im going back with my phone tomorrow and if they give me a new one will try to avoid upgrade patches and see how that goes.