Yamaha DTXpress/DTXplorer/DTXtreme

Sounds wierd.  Have you tried more than one output source (i.e.
headphones in the front socket, line out from the 1/4" jack on the
back)?  Did it happen suddenly or was it gradual?  Were you twiddling
anything at the time?

I had a strange volume problem when I got a new PA.  It was caused by
the fact that the PA could accept a differential (balanced) input and
I used the left mono output from the DTXpress, but with a stereo
cable. This left the other end of the balanced input floating.  A mono
cable fixed the problem.

I assume you have tried turning all the volume controls up (not just
the master volume)?

There is a volume mode (practice & live) page 30 of the reference
manual, although if you have turned up all the controls, you should
have eliminated that.

Keith.