iPhone's quality control varies too much
I can sympathize with those that are frustrated with the music sound from their iPhone devices, no matter what the model is, you just got to be lucky to get a good unit, one that sounds with a punch, with clean rich sound, bright highs and punchier lows frequencies!
I'm going to Quote another post from this thread because the user described this sound delema perfectly!
The primary reason I switched to an iPhone was because the iPod feature and it's sound quality, but in the pricess I had to replace my iPhone several times and I discovered how much variation in sound quality there can be from phone to phone, iPhone to iPhone, same model
My GF still has 3GS, it sounds louder, but very harsh, the high frequencies can hurt your ears. We have updated the software to 5.0 but I couldn't notice any change in sound...
I got my first iPhone 4 with 4.x software with Verizon, I must mention the lighting speed of their 3G network. The sound of this iPhone, although not as Lou d as the 3GS, it was clean, punchier, deep base and clean bright highs, I used to delight myself when I connected the phone to my truck Audio system, but due to Verizon expensive plan for such a limited service, I decided to go with sprint...
At sprint I got a 4S, and it also exhibited the same sound qualities of my regular iPhone 4, deep punchier rich base with clean sharp highs. But the vibrator on this phone ratter, and for that reason I took it back for another 4s. That was a big mistake, since then I tried 2 more 4s, and finally went back to a second iPhone 4 with sprint and neither of the last 3 phones could match the sound quality of the first two phones
Now I am stock with a phone that I rather not used its iPod feature.
My conclusión is that the quality on these phones varies too much, if you got lucky and found a good sounding phone, then keep it because the next one my desapoint you!
I feel like the iP4 sounds cleaner, clearer and more defined than I remember my 3GS sounding, but maybe it's in my head. I don't use EQ.
It also seems that the sound quality of the iPhone 4 is better in certain curcumstances, There appears to me to be more of a punchy bass sound on kick drums and it's a little warmer. So to sum it up some of the harshness from the mids I experienced on the 3Gs have gone. Me and my dad notice straight away when we played something on the iPhone 4 it sounded punchier and warmer.