Music isn't playing through headphones Latitude E6400

I've been reading through some suggestions, but they haven't worked, or aren't for my computer.

Hello. These instructions are for your model.

The first thing you want to know is whether the speaker hardware still works. You can test them by using the Pre-Boot Assessment tests. Your laptop has either the PSA version or the ePSA (extended PSA) version. Follow these instructions for using either test to determine if the speakers still work.

If you do not hear the test tones, then it is a hardware issue -- either the speakers have failed, or the connection is loose. To fix, re-seat the connection, and if that doesn't fix it then replace the speakers.

If you do hear the test tones, then the speakers are okay, which means one of these things: the audio is mis-configured, or the audio driver has developed an issue, or the signal switching circuitry in the headphone jack is not working.

So if you heard the test tones, download and install a fresh copy of the IDT audio driver. [Note -- this driver is meant only for the supported operating systems on the Latitude 6400, which  are XP, Vista, and Win7. If you are using an unsupported OS such as Win10, do not install that driver, and reply back and say which OS you are running.]

Installing the driver will fix any mis-configuration issues, and fix any corrupted driver files. Test the audio again at this point. If the speakers still don't work, then it is probably  because of failure of the switching circuitry -- that means that the headphone jack falsely thinks a plug is in the jack, and so it is preventing the audio signal from going on to the speakers. This problem does not affect the PSA/ePSA tests, so that's why I had you perform it first.

The first section of the Headphone Jack FAQ explains the switching-circuitry issue that I named "sense pin failure". The fix for that issue is to replace the audio jack circuit board. There is an address in the FAQ, where you can get the part for the E6400.