I have an iPhone 4, since the upgrade to iOS 5 the music does not sound right, the bass sounds are really distorted.
I did a test playing one song in iTunes, with the same earphones, then copying it to the iPhone and the difference was huge. In iTunes the sound was perfect.
I've seen some comments about it on the internet, recommending to enable "Sound check", but that does not solve my problem. I have tried different EQ settings but all with the same result, so currently I have it on 'None'.
Usual questions.. does it happen to anyone else? any solutions or workarounds recommended?
Check your music files on iTunes, left click and select Get Info, select the Options tab, you'll see a Volume Slider, maybe it's been moved to +100% somehow.
Had the same issue on iPhone 4S after upgrading to iOS5, seemed to have fixed it for me.