coachrice,
What you are asking is difficult to answer due to the fact that you have a used bike and don't know what the previous owner did to it.
1. make sure CB is off if the bike has one
2. Make sure the display shows a speaker icon, and not a headphone when you switch to external speakers.
3. Turn the intercom off.
After you have done all that, if you still have audio in the headset, and not through the speakers, it is probably not a setting that is wrong. You have an electrical problem.
Since you have rear speakers, take off the seat and disconnect the 4 pin plug that goes to the rear speakers. This will eliminate the rear wiring a speakers as the cause of the problem. If even one wire or speaker is shorted, it will cut out all 4 of them.
While you have the seat off, look around and try to determine if the previous owner has been doing a lot of electrical work. He may have installed an aftermarket amp, and it may either be defective, or he may have removed it, leaving the speaker wires from the radio disconnected.
Except for the common problem of rear speaker wires getting pinched, no audio on speakers only is not a common problem. Go through the checklist first and then disconnect that cable, and get back to us.