i have had the same problem in the gp2x days, most of the time the only solution was the build the libs yourselve, it could be you will also have to recompile other libs that depend on SDL_Mixer if there are any.
i do remember that getting sdl_mixer to use libtremor doesn't always work right away and you'll need to experiment a little with the flags.
Another thing which i don't how the situation is like on the wiz / canoo, is that on the gp2x i used to compile my games staticly always, since i used other libs / my own build libs, and not doing so wouldnt use the libs you compiled since it will load the libs dynamicly which are availible on the device itselve. so you might need to include the libs also or get gph or whoever created the prebuild toolchain to use them as well so they become a standard. This might not easy so i just compiled staticly.
Not many dev's like this, but it's the only solution at the time i knew that would work, would like to know what the situation is like now on the wiz / canoo with this, since i ordered a canoo myselve now and need to get up to do date