Please don't take the following as gospel as I could be wrong, however, I'm fairly certain this may be the cause
The right earbud acts as a receiver and transmitter for the entire system. As well as the speaker driver and battery, the right earbud also contains the Bluetooth receiver and radio transmitter. The left earbud, if you've not realised, does not receive an audio input unless in near vicinity to the right one when it is receiving Bluetooth audio input. I am fairly sure that the left earbud works via receiving a radio signal from the right-sided one, and is not connected in any other way.
The reason for a quiet left-sided earbud could be from two possible reasons related to this fact:
- The transmitter is in the right-sided earbud is damaged/faulty
- The receiver in the left-sided earbud is damaged/faulty
The only other reason for this particular type of fault could be a damaged driver.
Saying all of this though I can say in confidence that it is not economically recommended to attempt to diagnose or repair this product as it is outdated and extremely difficult to make the reconstruct. It would make more sense to purchase another set of earbuds as it would be much cheaper and time-efficient.