Unbalanced speaker volume (top too quiet)

I have had a white Z5 Compact for a week and have now noticed that the bottom speaker is much louder than the top, ruining the stereo effect. I am sure it did not come this way out of the box. I didn't notice until after the recent firmware update (32.0.A.6.152), but cannot know whether this is the cause of the problem. I went through the hassle of factory reset and the phone still has the problem. I haven't subjected the phone to any physical trauma at all and will be returning it.