If the headphones have sound in only one ear or differing sound levels in each ear or the sound in general is not what you expected then it needs to be determined where the issue lies - with the headphones or with the audio source.
NOTE: If your headphones feature a removable cable ensure that the cable is securely in place in the headphones. Many of the removable cables feature a twist to lock function and if the cable is not securely in place there will connectivity issues. In instances where there are two cables available (ie: a plain audio cable and an Apple compatible cable) try the alternate cable to ensure the issue is not cable specific.
The first thing to do is ensure that you have the audio plug of the headphones connected securely into the audio device. A properly connected cable should not wiggle. If the plug wiggles that could indicate a loose connection. NOTE: This could be a result of the cable not being plugged all the way in or it could be a faulty input or sometimes if the device you are connecting to has a case on it the case will prevent the cable plugging in correctly.
The next thing to do is to check and make sure that there is no debris (ie: no dust or lint) in the headphone jack of the audio source that may be inhibiting connectivity and causing sound issues.
The next thing to do is to check that the connecting jack is clean. If oils from the hands gets builds up on the plug or sweat evaporates on the plug the salts and grime may prevent proper contact from occurring. The connecting jacks can be cleaned using a basic alcohol wipe.
The next thing to do is to try the headphones on a different audio source to check that it is not the original audio source (or a setting on the audio source) that is creating the issue. If you connect to an alternate audio source and the issue is not there then you know the headphones are fine and the original audio source is the issue. NOTE: Many modern audio devices have audio limit pre-set at the factory and these limit need to be changed to get the full audio signal out of the device and to the headphones.
If it is determined that the headphones are the issue please contact Sennheiser directly for steps on how to proceed forward.