If it's what I think it is, it's par for the course when it comes to editing audio. When you make a cut and join two pieces of audio, their waveforms usually don't match up; the jump in amplitude or frequency causes that click.
The only perfect way to cut audio is to cut on the exact sample where each waveform passes the zero mark in the waveform, not the crest or trough. This is overkill, and entirely too time consuming unless you're creating audio loops or high end studio mixes.
The more practical thing to do is to finish off the mix with a little two frame crossfade on each clip. Clears the problem right up.
Angelo Lorenzo
Fallen Empire - Digital Production Services