It's a very long time since I used Audacity and my memory is very poor. However, I seem to recall that, somewhere, you have to tell it what the 'line in' option is. The default option wasn't, IIRC, the intuitive one - but I forget what it was, and what I had to change it to. The chances are, if you've upgraded from a previous version, it's overwritten the settings in the previous version you had installed, and you need to change them to suit your environment. Sorry that's a bit fuzzy - but look around the options, see what you can find.
HTC U12 Plus review: Hands-on