Q: there is a red light in my audio jack

    Remove the headphone.

    If you see red light in the headphone port, plug the headphone

    in and out of the port 5 or more times to flip the microswitch inside.

    Sound output may be stuck in the digital mode.

    Best.

The headphone socket switch is stuck. Insert and remove your headphones a few time to see if the red light goes out. The red light mean Optical Audio output.

If the afore mentioned suggestions fail, try blowing hard into the audio output port.  Use compressed air if you have it.  If that fails, use a toothpick and jiggle it inside the port.  If that fails, make an appointment at an Apple store genus bar.

Ciao.

Background, or "why this happens": the audio-out port on a modern Mac is digital and is held to high dimensional standards. Most headphones have an analog plug not made to such standards. The specific issue is the the "tip-to-stop shoulder" distance on many analog plugs is just a bit longer than on a digital one and jams the "mute" contact inside the port in the muted position.

The best action is prevention. When connecting headphones (or external speakers), never push the plug until it stops until you've determined that the plug is properly dimensioned. Rather, put on the headphones and have an audio track running BEFORE you insert the plug. Insert the plug slowly, listening for sound on both channels and, when you hear it, STOP INSERTING.

Bottom line is you do NOT want to insert any random plug "to the hilt" until you know it is dimensioned to fit a digital port.

My MacBook Pro has the same jack. I have a pair of decent-quality, name-brand headphones and their plug is not to spec. This picture shows their plug in the port at a position that delivers proper sound and does not let the plug fall out:

Note that the shoulder is about 1/16-inch from contacting the computer case. It is secure in this position.

Every plug is different. I have a pair of El Cheapo external speakers I picked up for US$2 at a yard sale and their plug fits perfectly on my Macs with a digital audio port. Go figure!

After fighting with and destroying some toothpicks, I used a tiny 1.3mm crochet hook to flip the switch inside the port from the 3 o'clock position toward the center of the port. The switch gave a satisfying click and the red light went out. The sound is coming out of the internal speakers just as it should. So, if you know any crocheters or lace knitters, the tiny lace hooks and needles work like a charm. And I am much happier than I was a few minutes ago when my MacBook kept giving me the "redeye" while I was struggling to fix its sound.