I have two Nano-lookalikes, one works perfectly, the other has problems with audio - ie. the music is there, the chorus is there but male vocals are very faint, also, audio-books are inaudible. The only difference between the two MP4's (incidentally bought at the same place & time) is the firmware version, the working one has CTW500_S75, the poor audio one has AK1025. Video works but with no audio. Any ideas would be greatly appreciated.