The 'mixer' button let you change any of the 16 channel tones (using the cursor to toggle thru the channels...the one you are 'on' will flash, on the display). The first 4 channels can be 'toggled thru', just by pushing the 'left/right' cursors. If you press the 'tone' button...the display will show the current tone for the channel that is flashing. If it is set to a drum (500 to 515)...then that channel will play drum sounds. If you punch in (as an example),'000' ...the tone will change to the piano. Just be sure to press the 'tone' button first...to display the tone selection.
On my Casio, when I play a SMF file...the channel that my keyboard is using (for me to play along with)...is 'channel 4' (you can see the 'bar' move when you play a note). The channel you see moving the bar, is the one which you need to set a 'musical' tone, like the piano.
Also, check out this site for more Casio wk helps: http://www.originalz.com/casio/casio_bbs/
[This message has been edited by Jim Wintringham (edited 08-10-2005).]