If one earphone has a lower volume level than the other:
There is a chance that the audio file is damaged. If the audio file is damaged, it may give the effect that volume is lower on one earphone. Try multiple audio files to ensure this isn’t the problem.
The device may have an issue.
Test your device using other audio devices. This ensures whether it is the device or the M1s that are the source of the problem
Try getting a better connection by reconnecting after powering the M1s off.
Try factory resetting your earphones:
1. Place both headphones in the case.
2. Press and hold both buttons until the earphone status lights briefly turn blue.
3. At this point, you can release the buttons as the reset procedure is complete.
4. The earphones will no longer remember any of the devices that have previously paired with them.
5. You will then need to follow the initial setup procedure from scratch including the manual Bluetooth pairing process, achieved by holding down the earphone buttons for 3 seconds to enter pairing mode.