I checked images of the On-Tour speakers and it is clear that the mini-plug is the tip/ring/sleeve configuration of a standard stereo connection.
However, I believe some Apple audio jacks are tip/ring/ring/sleeve. If that's the case with your machine, there may have been a mismatch.
An observation only but the tip/ring/ring/sleeve jack is designed to be compatible with either a tip/ring/ring/sleeve or tip/ring/sleeve plug so that would be an unlikely source of the problem. There could be a conflict inserting a tip/ring/ring/sleeve plug into a tip/ring/sleeve jack. However having inadvertently done that more than once the only ill effect I have encountered was a very loud and annoying buzz from the speakers.
I suspect plugging in the speakers was only coincidental. I thought Ira's suggestion to reset the SMC would work but since it didn't I only have one suggestion. Back in the day there used to be a sovereign
solution of doing a complete shut down including disconnecting all external devices, disconnecting the external power supply, and in the case of laptops removing the battery and letting every thing sit for anywhere from 15 seconds to three minutes before putting everything back in the reverse order in which it was disconnected and then rebooting. This allowed some hardware registers to drain down and sometimes cured otherwise intractable problems.
Since it does not cost anything, it would seem to be worth trying, but since the battery in your MacBook Pro cannot be removed you will have to bypass that step. 😀