I believe the problem is that you are trying to Sum up what essentially is text. 01:43 plus 00:44 doesn't make sense because Access thinks they are both text and not time.
What you need to do is change duration to this:
DateDiff("n",[StartTime],[EndTime])
If you sum that field it will calculate correctly. Of course the result will be 147 not 02:27. For that you'll have to build a custom formatting function.