Sound problems can be infuriating! Some options:
Look in the controls for your media player and see if you can set its output channel. That's a crude solution, but at least it's a quick fix.
See if you have pavucontrol available - it's launched from the command line - and if you haven't, get it. It should enable you to set output.
If that fails, then use the command
With everything plugged in. It should list things like
card 0: Device [C-Media CMI8738], device 0: CMI8738-MC6 [C-Media PCI DAC/ADC]
card 1: Device [USB Audio Device], device 0: USB Audio [USB Audio]
The internal sound and the speaker jack may use the same device or not: sometimes the video card may be used for sound. The usb device, when plugged in, should definitely be listed. If you don't get the usb device listed, see if the driver is there with
If you don't get snd_usb_audio in the output, it's missing and need to be installed with the package manager.
If necessary come back to us with the output of aplay.