I have two imitation Nano MP4s. One works perfectly but the other has poor audio - ie. the music sounds OK, the backing vocals are OK but the lead singer is very faint and audio books are inaudible. The only difference between the two MP4s (incidentally bought at the same place & time) if that the working one has firmware version CTW500_S75 and the poor audio one has AK1025. Any ideas would be greatly appreciated.