Good tip. I have a set of Bluetooth speakers (that seem to work fine) with a plug in. Will try that. I have the phone volume set to 10 for playback. Will try another cable if nothing else works, then maybe think about seeing if the sound levels can be changed on the mp3's post production.
Scott--also try a different device on the aux/in, if you have another ipod or phone or whatever you have with a stereo jack that has an out port laying around, if you haven't already.
Try a iPod or MP3 player and see if that changes it. i know phone volume and iPod Volumes are different. that is why it sounds OK in ear phones and the phone asks you if you are o.k. when you try to turn it up.??? but other than that ground issue or a stereo cable could be causes...but this thread was a great read you do have to admit. Chad Detroit Tuned (not tunes)