That sounds like a hardware issue. For example:
- You have blown your speakers by playing too loud
- Your speakers are physically damaged
- There is a wiring issue – electrical short or wired to the wrong terminals
I would open up the laptop and check it out. You can use videos and pictures online to check the speaker connector polarity (which colours go to which pins) on the motherboard, and you can remove the speaker housing to check the speaker membranes for any visible damage.
I’m not sure what kind of warranty Teclast offer, but my personal understanding is that once you buy China you’re on your own with no help whatsoever, except of course from the lovely people in these forums!