With recent fully wireless earphones, sometimes the level in one channel will be much lower than the other, and there seems no way to fix it. Fortunately, there is a reason for it, and it is fixable.
Earlier earphones/headphones used a wired stereo connection from the source device to one of the earpieces and from there another single channel connection to the other earpiece. Latter ones had separate wires direct to each earpiece.
Early Bluetooth earphones/headphones simply replaced the source wire with wireless, while keeping the wired connection between the earpieces. Later, the wired connection was replaced, but the stereo connection from the source was still to one earpiece. These configurations typically don't have level mismatch problems.
Modern Bluetooth chipsets use True Wireless Stereo (TWS) connection technology to use separate wireless connections directly from the audio source to each earpiece. They are completely separate, to the point where each earpiece can be connected to separate source devices. It is these separate and independent connections where the potential to have different levels arises. This is because there is usually a delay between when each channel connects, and it is in that gap that adjustments to the first connection's level can lock in level imbalances.
Typically those adjustments are a result of a quick panic to turn down a level that is too loud, while the second connection curiously gets the loud level when it connects right after the adjustment. This seems to be a bug in the protocol, and is possibly because each channel is treated as a mono device, but that just happens to be used as a stereo pair.
The first channel appears low, but it is at the low level it was quickly adjusted to. It is the second channel that can be very loud. So the audio source volume value is shown as low, but there is no way to increase the first channel to a reasonable listening level without putting the second into distortion.
Note that when TWS-capable earphones/headphones connect to a non-TWS-capable audio source, there will be a properly balanced stereo connection to one earpiece, with a separate connection with the other, just as if there were no TWS. There is no opportunity to be imbalanced.
The long-term solution to preventing panic adjustments is for the earpiece manufacturers to program their devices to gradually increase levels from zero after connecting, so giving time for both earpieces to get connected before a user needs to feel they need to lower levels.
So now that we have the reason why the level disparities occur, what can be done about them?
Firstly, as a preventative measure, always keep the volume at zero or a known safe level before making the Bluetooth connection. That will mitigate against panic adjustments. Fixing an imbalance is what comes next.
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To fix TWS Bluetooth level imbalances:
1Totally isolate the earphones/headphones
by unpairing at their end, and powering them down, typically by placing them in their charging box and following the unpairing instructions.
For example, Nillkin TW004 earphones require putting them in their charging box and tapping each of them 5 times.
2Make the source device forget about the earphones/headphones
by unpairing each earpiece in their Bluetooth settings.
The earpieces are no longer listed as paired devices.
3Set up one earpiece
3-1Pair the earpiece
by removing it from its charging box, leaving the other one there, and pairing it to the source device.
Sometimes the other earpiece may also be shown as paired or connected, but just unpair it in the device. It is just a phantom listing that got remembered.
Unfortunately, pairing can sometimes be a rather hit-and-miss affair, so repeat this whole step process until it works.
The earpiece is now paired, and the source device will show that, while the earpiece may annouce it.
3-2Connect the earpiece
typically by clicking on a Connect button on the source device.
The earpiece is now connected, and again the source device will show that, while the earpiece may annouce it.
3-3Set the Bluetooth volume to zero
in the audio app being used for listening.
When applied to each channel separately, this makes sure that both levels are matched as precisely as possible. Some source audio devices don't show precise volume level values, but just a short bar, so trying to visually remember the lengths of those bars between each channel's setup may not result in matched levels.
3-4Isolate the earpiece
by disconnecting and unpairing it at its end, and placing it back in the charging box.
4Repeat exactly the same process for the other earpiece
including placing it back in the charging box.
5Pair the earphones/headphones as a stereo pair
by patiently following the prescribed pairing processes for the earphones/headphones and the source device, without making any level adjustments from zero until both earpieces are connected and paired.
The volume from each earpiece should now be matched, and subsequent pairings will typically automatically happen without a hitch, as long as no levels are adjusted until both earpieces have connected.
As is often the case with smart devices, they are programmed to cater for us poor humans by remembering settings and configurations from the past. This may mean that this procedure may be need to be repeated to clear out those programming gremlins.