Showbox has 2 params
setshowbox "file-1.sbx","file-2.sbx"
File-1 is used for all but samples (notice that music is also in file-1.sbx) and "file-2.sbx" is used for nothing but soundfiles (wavefiles and so, music is on file-1.sbx)
I assumed as music was a sound it was in the 2nd file but no.