First of all, it would help if you mentioned which box you are using. The BT400 comes with many boxes.
Since you reset the remote, do you actually see a volume bar now on the screen when you try to change volume up/down ? If not, can you check if you have "Force MAX volume" option enabled ?
You say you connected a wireless speaker, but you don't say if it is paired to the box, or your TV ? Have you tried using the TV remote to increase the TV volume ?