1) If your times add to more than 24:00, you need to format your result
with Format/Cells/Number/Custom
[h]:mm
to keep XL from "rolling" over the time.
2) If your times are text, SUM() will ignore them. Copy a blank cell,
select your times, and choose Edit/Paste Special, selecting the Values
and Add radio buttons. This will coerce text into numbers.