I got my hands on the QCY QS2 wireless earbuds a couple months back and noticed the audio to be rather quiet out of the box. This was the case even at higher volume levels. If you have a device that runs Android then the following may resolve your issue! (YMMV)

Under the "Networking" section in "Developer options":

  • Change the "Bluetooth AVRCP verison" to 1.6
  • Turn on "Disable Absolute Volume"

You might have to restart your mobile device after as well.