So the first time you go to a restaurant, do you say to the chef, 'hey, I've never eaten your food before, so maybe I could have my first meal for free? Then I'll judge for myself whether it's worth paying for'
Or when you need a plumber or an electrician, do you ask for your first job for free because you've never used them before?
I know its not the same. But it is the same. There is a culture of work for free in the entertainment business which you don't find in other industries; its why musicians struggle to do their craft and pay the rent.
And I totally understand about building a portfolio, I get it. I've worked for free to build mine. But I've been doing this for 10 years, and its still not my main job, I hold up a desk in the day to pay my rent. What if I'd been paid for all the jobs I did for free? Maybe I'd have a viable career. And if people didn't like what I did for them - they don't have to use me again. You wouldn't go to a shit restaurant twice, would you? But you'd still pay the first time you went.
My advice would be, seek out work for your portfolio face to face. I'm realistic; I know you have to work for nothing in this game. But don't advertise it. Go and see some bands and approach them directly - 'I really like your stuff, and I'm building an engineering portfolio. Do you want a song recorded for free?'. Build face-to-face relationships with artists. Hassle some studios to sit in on some of their sessions. You never know - you might start making coffee there as a favour, and then get the opportunity to mix a song if they're ahead of schedule.
You have to be pro-active - and posting on forums saying you'll work for free is just another way of telling yourself that you're doing something to further your career, when in actual fact you are just getting lost in the thousands of other engineers who are willing to work for nothing and thusly not doing yourself any favours, whilst contributing to the problem.
Sorry if this comes across as harsh, but in my experience, in my country (the UK), it's true.