Are you sure they are plugged in all the way? Sansa players are notorious (the past several years) for having extremely tight headphone jacks when new. When they’re not fully seated, they only play out of one side.
If they are truly broken already, you might try the Sansa Store for replacements. Or any one of a million other options out there; your earphones/buds don’t have to be OEM. In fact, many consider them cheap and inferior-sounding in comparison to all the other phones on the market these days.
