Hi Ciireenee,
I've been scouring the net and I can find plenty of people affected by this issue, but no fixes I'm afraid.
As I said previously the fault appears to be purely software based, as I've actually tested a different headphone jack cable.
Since it's software the only options would be to (1) downgrade your phone to iOS 4, (2) return it Apple to exchange it for a replacement phone, or (3) wait for Apple to address the problem and fix it in a future update.
Since Apple do not let users downgrade to previous versions of iOS option 1 is only possible if you have previously taken measures to backup your SHSH blobs, which I'm guessing you haven't done. Option 3 is a possibility, however at this stage I haven't found anything to indicate that Apple have even acknowledged this issue, let alone made any efforts to address it. As such as fix for the issue in a future update is unlikely. This of course leaves you with option 2.
Given that many people are affectd by this issue I wouldn't go down without a fight. Even though your warranty has expired I would pressure Apple to replace your phone on the grounds that it was rendered faulty by their software update, not through any mistreatment of the phone.
To support your argument, I would scour the various iPhone forums out there and print out all the threads you can find of people describing the same or similar sounds faults following the iOS 5 update. Once armed with this evidence I'd book a genius appointment and take your phone back.
If Apple wont budge and refuse to replace your phone, as a previous iPhone owner you are entitled to exchange your faulty iPhone (even though Apple actually broke it!) for a replacement phone at a substantially reduced cost (compared to buying a new one). The other option of course is that you retain your faulty iPhone, hold onto your money and then send Apple a message through buying an Android phone. Hopefully it wont come that though, in my experience Apple are extremely generous when it comes to keeping their customers happy.
