By the way, Sony support have come back to me. They aren't aware of this problem and aren't going to issue a fix for it. I therfore suggest you get your money back:
"Thank you for contacting the Sony Xperia support centre.
We are sorry to hear of the problems you are experiencing with the speaker on your Sony Xperia L.
We can advise the fault mentioned is not one known to Sony and have therefore not released a fix for this problem in a software update."
