You’ve probably already done this, and you say you get sound from headphones, but (on the Mac) check:
<System Preferences> <Sound> <Output>
With the lead already pledged into ST3, check that the output is set to “Headphone port” (and that that’s the one you’re actually plugged into, not the mic/line input)
Check that “Mute” is not ticked, and that the volume is up to a good level – over half way at least. Also that “balance” is in the middle, just in case.
Finally, check that your music is still playing, as the computer will, I think, sometimes pause playback when you plug things in & out of the headphone port. This is where I hope the problem may be…..