This is by no means a comprehensive study of price/ volume relationship but it is interesting for general observations. We are told not to trust low volume rallies. I believe that interpretation of volume is more important for individual stocks than in the broad market averages. Either way, most of your attention should be focused on PRICE. Only Price Pays!
click chart to enlargen the weekly chart of $SPY
Green shaded areas shows flat to advancing volume, red areas indicate periods of flat to declining volume.