in testing the specificity of enzymes action, what do you think are the roles of NaCl & the phosphate buffer?

Answer 1

does it not alter the pH of the solvent? if the pH is altered, the enzyme may become denatured, inhibited or may work optimally

Answer 2

You are not providing enough info.
Buffers are used to ensure optimal enzymatic activity since the latter is pH-dependend. I am not sure whether buffers play a role in the specificty. Sometimes you can use different buffering systems for the same pH value. Provided that the buffering ability is not compromised, you might observe different enzymatic activities since the ions of one of the buffering systmes could be activating/inhibiting the enzyme. Still, this is not related to the specificity.
Of course if the substrate has groups that ionize in solution, the pH will determine the charge of such groups and thus affect its ability to bind to the enzyme. However the effect of pH on the enzyme itself is usually so significant that usually it is much greater than the effect of substrate binding so you can't really use the effect of changing pH to draw a conclusion about specificity.

Ionic strength on the other hand can play a role. If you increase the salt concentration too much you might even have denaturation of the enzyme and precipitation. For moderate changes, increasing salt concentration (ionic strength) makes the hydrophobic interactions stronger and the ionic weaker.
Depending on the nature of your substrate and its interaction with the enzyme, the specificity might increase or decrease. Usually ionic interactions increase/contribute more to specificity thus in most cases I would expect a decrease in specificity.