If the sound is not quite what it should be, do not forget to check your cord connections. You should try using the jack in the other earpiece of the headphone. Also, you might try plugging the headphones into a different device to rule that out.
Your headphones may be uncomfortable if one or both of the ear cushions have fallen out, or are coming away from the earpiece. This repair may involve using glue, so be sure to follow the manufacturer's instructions first. In addition, a needle and thread may work in other repairs. If repairing doesn't work, then the earpiece may need to be replaced.