Hi, i bought my Bv6000s on january 2017 on ebay store, but the last month the loudspeaker began to be heard very low, hardly hears the call bell. I tried to contact the seller of ebay many times without getting a response and I do not know what to do. The phone must have a one year warranty as indicated by the seller.
I have tried what appears in the youtube video, immerse it in water, pass an iman through the speaker but nothing, I can not increase the volume.

For the warranty contact your seller. If all the things with water and magnet don't help you could buy a new speaker very cheap on platforms like aliexpress.

you can also try download the app on playstore, "MTK engineering Mode Start"
from here you can adjust the speaker volume limits. try tweaking it a little higher.
My speaker is also crap. I have to use vibrate for calls or I dont hear it

Buy new main speaker for 2-4$ or replace whole rear battery cover with built in speaker (in your color device) for 15-20$.

If your choice will be change only the main speaker you must search for waterproof silicone to glue it. Somewhere on this forum is a guide how to replace the speaker you can find it.

All those suggestions are mostly nonsense. (Especially the MTK engineering mode thing. Please do not ruin your volume settings. You will not remember what it originally was!)

Please do not listen to them, unless you have tried these things first:
http://bbs.blackview.hk/viewtopic.php?f=142&t=326558