If might be helpful to provide a more detailed description of the problem, for someone to offer useful advice..
It sounds like you're re-linking an audio file? A quick search suggested that you might want to check to make sure both the file format and the channel format (mono, stereo) is the same as the original..
I'm an audio guy myself, so I can't make a more detailed suggestion, but I've linked the thread to a FCPX course [above], in case there might be something in there that would help..
Also, here's the Apple support page on re-linking files in FCP: