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. The following may resolve your issue but 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.