When you are trying to get sum of a TimeFormat, you should sum the minutes.
Try executing this query in your query window
Declare @TempTable Table(TimeValue VarChar(20))
Insert Into @TempTable Values('12:00 PM')
Insert Into @TempTable Values('1:00 PM')
Insert Into @TempTable Values('10:00 AM')
Select Sum(DateDiff(Minute, 0, TimeValue)) As TotalDuration
From @TempTable