You were close, try swapping you BG positions around.
If I understand right, you want the top blue portion of the sprite showing as the default state
.playButtonb {
background-position: 0 0;
}
.playButtonb.active {
background-position: 0 -260px;
}
Be aware that the class name .active can easily be confused with pseudo :active
Try to select class names that don’t conflict with established html/css elements and properties