If you're comfortable doing so, you can take apart your controller, find the little speaker (it's on the bottom of the bottom module), and stuff the hole with cotton or something like it (I used Sugru). It's pretty easy, probably won't void your warranty, since you rarely have to send in your remote when you're looking to repair or replace your aircraft, and works a charm. You can find video instructions on Youtube.
There is a thread about it .
It needs a little work on the controler, but it works great.
I cant find it right now, but 'll try to recover it for you later...
You only need to make a little hole in the RC bottom, just above the bipper (the exact position is showed in that thread), and then you only have to insert a little screw in that hole, which screw down to the bipper. So you can adjust volume by screw/unscrew ...
I did it on my RC and it's just perfect, works really great. I adjusted it and now I can hear a very low bipping!!
I think the remote volume is controlled by the volume on your phone or tablet that you are using. If you turn your phone volume up or down the FileZillaMalwarebytesRufus volume from the remote goes up or down