EVERYBODY WHO HAS READ/SUBSCRIBED TO THIS POST, PAY ATTENTION!!!
rschnei... you're on to something with the Late Night EQ!!! The Late Night EQ IS THE PROBLEM for the low sounds with iPhone 5 & iOS 6. Instead of doing a hard reset try this:
1.) Go to Settings ---> Music ---> EQ
2.) Starting at the top of the EQ list with Acoustic and working your way down to the Late Night EQ, select Acoustic and immediately lock the iPhone. Select Bass Booster and immediately lock the iPhone. Keep doing this with each EQ setting going down the list WITHOUT skipping EQ's. Once you select the LATE NIGHT EQ and lock the iPhone, notice how LOUD the lock sound effect gets!!! There is a noticable difference to the ear!!!
I found that by selecting and leaving the EQ setting to 'Late Night' on the iPhone, ALL sounds seem to come in loud and clear. For some odd reason if you go back to the Music setting and select any other EQ ALL sounds seems to revert back to being on the low side. Again, you DON'T have to do a hard reset for this to work. Just select Late Night EQ and that's it!!! I really hope Apple fixes this bug...
PROBLEM SOLVED!!!