I am in the exact same boat in regards to these time metrics and am probably an even newer user than you. Have you had any luck yet? I currently have the metrics calculated in the DB, however am having the same trouble when bringing into Tableau.
Hopefully someone out there has a suggestion.
What I ended up doing until something better comes along is to us a decimal representation of the time. As an example 1:30 or 90 minutes would be 1.5
0:45 or 45 minutes would be 0.75
I found a solution that works well at the day level, which is where my SQL view is aggregated....however having some trouble weighting the aggregation for the roll ups, however it may work if going after the raw data....what I do is the following:
Seperate the Minutes
(FLOAT ( Left(STR ( [Average Call Duration] ),2))))
Seperate the Seconds
((FLOAT((MID(STR([Average Call Duration]),FIND(STR([Average Call Duration]),".",1)))))*60
Put it all together
I'm still looking into other options.