I can only speak for my own experience but I've certainly heard many different ways people have ended up in post-sound.
I did a BA in music and pop music where I focused on composition and music tech. At the end I didn't know what to do, certainly didn't want a normal job unrelated to music. I heard about an MA in film music but was rejected. However, the professor also ran an MA in Sound Design and thought I'd do well. I got my MA and a wife from the course and moved to Belfast where I did short films for free until I started getting some paid TV work. I ended up doing everything from ADR records and editing to mixing entire TV shows in my spare bedroom. A few years ago I got a full time job as a mixer in a post-production facility.
I have to say that I went in to the job thinking I knew a lot. Even now I am still learning, which I have come to understand is a good thing.
There are some things I would never have known not working in a facility with people to tell you how things work. Take PPMs for example. I'd never heard of them until a short film I mixed failed the QC when it was mastered to tape. A quick bit of research and a remix later it passed and the director couldn't tell the difference.
In hindsight I have to say that learning about sound in uni did wonders for my knowledge and passion. Actually working in the industry for myself then in a company has honed those things and added experience into it. I think I've said before on here (although it might be the DUC) but I find that taking from many sources is the most beneficial method. I've learnt a lot from teachers and collegues but also from listening to films and finding my own way and making mistakes.