So you stick the jack halfway in and then wriggle it to get the on-board speakers to work? If so, then, clearly, the on-board speakers think something is still plugged in even though there isn't. Can you repeat this problem?
Mine have been fine from day one and I'm constantly switching back and forth
This is the first instance of something being physically wrong with the audio, so let's see if we can get to the bottom of it before sweating it....