John's reference to the 114 to 126 volt standard is correct, although that pertains to the voltage at the building's electrical service panel. Voltages at your outlets can be as low as 110 volts on a sustained basis and still be consistent with the standard, and may drop to considerably less than that sporadically while still being consistent with the standard. See this reference
Also, a point to keep in mind is that most of the Furman conditioners which provide voltage readouts have a specified accuracy of +/- 1.5 volts. And a 1 volt fluctuation in the reading, corresponding to its resolution, can be expected to occur at times even if there is no change in the actual voltage.
That said, it is certainly possible that the sound quality of your system is being affected by both the relatively low voltages and their fluctuations. Especially (but not only) in the case of the power amplification circuits, which in most designs are powered by unregulated DC voltages, which will therefore vary in proportion to the AC line voltage. However, the resulting effects on sound quality figure to be highly component and system dependent, and to not have a great deal of predictability.
So the first thing I would do is to take some time and try to assess whatever correlation there may be between the various voltages you see indicated and variations, if any, in the sonics of recordings you are very familiar with.
If you do sense a significant correlation, a power regenerator as was suggested would definitely be a solution worth considering, albeit a somewhat expensive one. That would also minimize or eliminate any adverse sonic effects which may result from noise and distortion on the AC line. But I would add that reported experiences with regenerators are not unanimous, with some people reporting considerable benefit but others reporting compromised dynamics or other issues. As with a lot of things in audio it seems to be component and system dependent, without much predictability.