Hi @zage12 ,
If the left speaker doesn’t work when using either the Bluetooth or audio cable connection first check that the left speaker is still connected to the circuit board etc.
Here’s a teardown video that shows how to open the headphones so that the speaker wiring connections can be inspected.
Here’s another video that shows how the same problem in the same model headphones was caused by a poor soldered connection associated with the speaker wiring.
You may not have to purchase any parts but initially would need soldering tools and skills to repair any faulty soldered connections and if there are none then perhaps a DMM (digital multimeter) would be necessary to further test the speaker circuit e.g. the speaker itself may be faulty.
Hopefully a start.