My speaker left audio jack not working

I'd recommend trying to isolate the issue and ruling out some easy fixes. I've detailed a few things to check below:

1. Try a different pair of headphones to rule out the possibility that the ones you are using are damaged or malfunctioning. You can also try connecting an external speaker to rule out a headphone issue.
2. Try resetting the PRAM/NVRAM.
3. Check your system settings
   1. Go to "System Settings" (little gear app icon)
   2. Click "Sound" near the top of the settings list.
   3. Under "Play sound effects through" near the top be sure to select your headphones as the output device.
   4. Under "Output & Input" select "Output" then be sure that your headphones are selected.
   5. Be sure that your "Output Volume" is properly set and is not on Mute or set to low.
4. Try using Terminal to restart your core audio.
   1. Press Command (or Cmd) ⌘ + Spacebar to open the Spotlight Search
   2. Type "Terminal" into the search and open the Terminal App (sometimes called Terminal.app) which should be the first result.
   3. Type sudo pkill coreaudiod into the Terminal window and press enter. (You may be prompted to enter your password)
      1. Sudo will run the command as an administrator (Su-Do as in Super User Do) and pkill sends a signal to the process based on the name that follows, in this case, the coreaudiod, which is the name of the process that controls your audio settings.
      2. MacOS will automatically restart the process after you kill it.

If all of that fails you might need to replace the headphone or audio jack. Unfortunately, Apple soldered the audio jack to the motherboard so if you want to do a DIY repair you most likely will need to replace the entire board. Alternatively, you can find a local repair shop that is skilled in microsoldering so that they can perform the repair for you. Alternatively Alternatively, if you feel confident, you can attempt to microsolder the replacement yourself.