In the event that a music system has speakers that vary in loudness, there are a few possible actions that can be taken to solve this problem:

First, check the wiring to make sure that all speaker cables are properly connected and none of them are damaged or loose.Then, check the balance settings to make sure they are set correctly. It is possible that the balance has been inadvertently shifted to one side.

The speaker position is also important. You should make sure that the speakers are positioned symmetrically and are at the same height. If one speaker is placed higher or closer to a wall than the other, this can result in unequal volume.
It is also important to check the speakers themselves.
It may be that one speaker is defective or has a damaged diaphragm. If this is the case, you should test each speaker individually by connecting only one and playing music. If you notice a difference in volume, it may be that the speaker is defective and needs to be repaired or replaced.

If all these steps do not solve the problem, one can consult a professional for further assistance and possibly repair.