Creating new table

Anonymous Thanks for providing the sample data and detailed explanation.

Here is the steps I've followed to achieve it

Step1 : Added new field in the source table as below

EOMonth = EOMONTH([Date],0)

Step2 : Created new table as below

CustVolOut = 
VAR _Temp = SELECTCOLUMNS(CustVol,"Customer",[Customer],"Volume",[Volume],"Dat",EOMONTH(CustVol[Date],0))
VAR _MinDate = MIN(CustVol[Date])
VAR _MaxDate = EOMONTH(MAX(CustVol[Date]),0)
VAR _Calender = FILTER(CALENDAR(_MinDate,_MaxDate),[Date]=EOMONTH([Date],0))
VAR _CrossJoin = CROSSJOIN(_Calender,_Temp)
VAR _Res = FILTER(ADDCOLUMNS(_CrossJoin,"Flag",LOOKUPVALUE(CustVol[Customer],CustVol[Customer],[Customer],CustVol[EOMonth],[Date])),[Flag]<>BLANK())
VAR _Res1 = ADDCOLUMNS(_Res,"Flag1",IF([Date]<>[Dat],"b","a"))
VAR _Res2 = ADDCOLUMNS(_Res1,"Volume1",IF([Flag]=[Flag1],[Volume],0))
VAR _Res3 = SELECTCOLUMNS(_Res2,"Customer",[Flag1],"Sales",[Volume1],"Period Ending",[Dat])
RETURN _Res3

Step3 : Add a new field in the table that was created in above Step2 

TotalCummSales = CALCULATE(SUM(CustVolOut[Sales]),FILTER(ALL(CustVolOut),CustVolOut[Customer] = EARLIER(CustVolOut[Customer]) && CustVolOut[Period Ending] <= EARLIER(CustVolOut[Period Ending])))

Finally, here is the output table..

I definitely believe, there might be more efficient way of doing the same, but this is the approach that is currently on top of my head. But I'll be glad if it atleast helps you a percent....