sum time duration

I am making a database of musical compositions each with several tracks
lasting just a few minutes and seconds . I want to be able to sum the
duration of the tracks for each compositon.
Please can you tell me how to do this? I have tried formatting the field
'Duration' as 'Date/Time' but how can I designate it as minutes:seconds and
not the hours:minutes of Short time? Then how would I sum the times?
I tried choosing short time and put an input mask of __:__but when I summed
the field I got a decimal number.
Thank you for any help you can give.