Even when you're careful about miking an instrument (or even an entire ensemble), you may find that on playback, one side of the stereo recording is not quite as loud as the other. You can open these files within BIAS Peak (now at version 6) and compare the waveforms. If one side (left or right) appears to be smaller, this indicates (at least in the majority of cases) it's not as loud and that a simple gain change will bring the quieter side's level up to match the louder side. Peak conveniently offers a Change Gain function under its DSP menu. But before attempting to pump up the volume of the quieter channel, click on the Clipguard button. This will assure that no part of the audio will be so loud that it will produce nasty digital clipping. In fact, when you click on the Clipguard button, Peak automatically sets the gain slider to the maximum allowable level.

Since the editing you are doing in Peak is non-destructive, you can choose to change the gain to the level indicated by Clipguard, then have a listen to the results. Usually this does the trick, but if one side still seems louder, it's possible that there is a time delay or offset between the right and left channels. In that case, you can go to a peak transient at the beginning of the recording and match up both sides by sliding one side forward or backward to line up the transients. If this works, it simply means that one mic was farther away from the instrument (or group of instruments). The ear will always interpret the earliest signal as the loudest, even if the volumes are almost exactly the same.