That's definitely unusual! For how long has this been the case?
Try to make sure the audio connection is clear of debris, so there's nothing causing a blockage. Have you checked the condition of the audio cable? There might be an issue with it.
Also, what I'd usually do is to try and test a different set of headphones, just to make sure it's not an issue with the audio you're listening to. I look forward to hearing back from you - let me know what you find!
DJT here. Thank you. I now find that if I hold the audio cable where it goes into the headphone in a certain way, then I get sound out of both headphones as it should be. But as soon as I release my hand it goes back to sound out of one speaker only.
Thank you for testing that out further.
What device are you currently using the SoundTrue headphones with?
Furthermore, what's the outcome if you try these headphones with a different device - do you notice any improvement or difference in audio output?