...
I may get a chance to beat Peo to answering this. You'll need either an array of
the worksheet names between Abba and Bjork or a range containing them. Order
isn't important. Then use INDIRECT and SUMPRODUCT in addition to SUMIF. Like so
=SUMPRODUCT(SUMIF(INDIRECT("'"&{"Abba","AC/DC","B52s","Bjork"}&"'!"&
CELL("Address",C19)),"M",INDIRECT("'"&{"Abba","AC/DC","B52s","Bjork"}&"'!"&
CELL("Address",D19))))
Note that this leaves C19 and D19 as fully relative cell addresses, so if you
copy the cell containing this formula and paste into the cell below it, the
formula in that cell would be
=SUMPRODUCT(SUMIF(INDIRECT("'"&{"Abba","AC/DC","B52s","Bjork"}&"'!"&
CELL("Address",C20)),"M",INDIRECT("'"&{"Abba","AC/DC","B52s","Bjork"}&"'!"&
CELL("Address",D20))))