Is there anything lodged in the port? it sounds like something is shorting to ground making it think that there is a headphone jack connected... Maybe a piece of the chrome plating from the headphone pin.
did you try replugging and unplugging again?
On old PCs sometimes the detector inside the earplug may not trip, and so the machine still "believes" the earplugs are there, so it just cut the loudspeakers...
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