I’m on Mint too. Here’s what I do to record audio playing on the PC.
Right-click on the little speaker icon in the taskbar in the bottom right hand corner. Then click ‘Sound Preferences’. Click on the ‘Input’ tab at the top on the window. You should see a box headed Choose a device for sound input. To ensure you record from the PC you have to choose ‘Monitor of Built-in Audio…’. Record the thing you want but remember to change it back to the default in case you want to record from another source.
I hope that solves your problem.
Mark B