I have the same problem but the audio works again when the headband is fully compressed, so I guess the issue is in the headband, not on the left speaker itself.
Did you try disassembling the headband to see how the sound travels from the right driver (which seems to be the one with all the master controls and probably the receiver for Bluetooth as well).
I haven’t tried to open it myself and was hoping someone knew how to do it without breaking them.