How does a lower pH change the alcohol concentration of wine?
This is a slightly broad question, but ill try to assist some. Google has a large assortment of articles describing pH, Le Chatelier's Principle, and EtOH. 

Im not sure if you mean altering the pH during the fermentation process or altering the pH of the final product? It also makes a difference on the types of yeast you use. Some yeasts can handle different conditions and produce a different amount of alcohol. 

Here is a great response from last year on the Stack Exchange, regarding fermentation without the addition of commercial yeast, using the change in pH as a catalyst for fermentation. 

With the production of wine that uses commercial yeast for fermentation (which is most commercial wines), altering the pH too far outside of it's "designated" range will cause it to produce less alcohol. Some connoisseur winemakers will alter the pH slightly sometimes, varying on the types of grapes, to alter the terpene profile present. Different variables, equals different systems of equilibrium.