[ubuntu] no sound through main sound-out jack

After installing Ubuntu 16.04 a few months ago, I had no sound through the main output (which I plug into my speaker system), but I had non-stereo sound through my headphones when I switched to them. After rebooting, it worked fine. Then recently the sound got rather choppy and started cutting out. I could get normal sound by half-unplugging the plug for my speaker system from the computer and jiggling it just right. Then after a couple days that stopped working. I tried plugging into the other sound-out jack on my computer, the one that's on the front -- but still nothing there. In the past few days I again have no audible sound output through the main system, but I get sound coming out through my headset if I switch to that output, though it sounds rather tiy.

Yesterday I tried ruing Ubuntu 16.04 from its installation disk, and sound worked fine while playing a Youtube video. Then I switched sound output to the headphones and back to the inteal sound system, and after that I could never get any sound through the main output any more.

My speaker system is fine: I got sound just fine when plugging the speaker system into my laptop.

How can I figure out whether the issue is my sound card or the operating system? And if it's a software issue, how do I fix it?