Sound Rises and Falls when using Headphones

The Macbook Pro 13" Retina allows me to easily connect headphones through it's headphone jack. Nothing appears to be clogging the port. The Macbook Pro has no issues switching from headphones to internal speakers or an external bluetooth Flip2. External sound works great. When I connect headphones and select a track to listen to whether it be iTunes, VLC, Quicktime, etc. the sound rises and falls. Usually the sound starts out quite loud and then becomes softer and softer. You can always hear something but not at an acceptable volume level. The headphones are new and work great when connected to other devices. PRAM reset doesn't help. I am thinking hardware but have no way to confirm. There are no "true" Apple Retail Stores where I live - just resellers. Any suggestions are very much appreciated. Thank you.