If your JBL speaker produces distorted sound, see the JBL Charge 3 Sounds Distorted problem page for possible problems and solutions.
Adjust the balance on the audio source (or connect the speaker to an audio source that has this ability) so that you can tell if it is in mono or stereo when you slightly withdraw the plug from the aux socket. Try full L and then full R to listen if each speaker still works.
It may be that there is a faulty aux jack in the speaker. Unfortunately, they cannot be repaired but have to be replaced
Here’s a link to the ifixit JBL Charge 3 Motherboard Replacement guide, which may be of some help gaining access to the aux port.
I cannot find a service manual or a parts list for the speaker so you may have to search on places such as this to try and get a compatible replacement part. Look for 3.5mm phone connectors and then check the datasheets for the component to see if it matches.
Hopefully, this is of some help.