HOW DO YOU CONTROL THE VOLUME ON THIS DARN...

Question

I JUST BOUGHT A MP4 MANBO-MANO....I AM NOT SURE HOW TO CONTROL THE VOLUME. EVERYTIME I PRESS THE "VOL" BUTTON ON THE BOTTOM RIGHT HAND CORNER....ALL IT DOES IS DISPLAY "A-B", "A-" OR "S"...WHAT DO THESE MEAN....IT STILL SOUND THE SAME...VERY VERY LOUD...W/ THE HEAD PHONES YOU COULD STILL HERE THE LOUD MUSIC.........THIS DARN THING DID NOT COME WITH A MANUAL....HOW DO YOU WORK THE VOLUME????