Pedro is probably right. Have delta time print to screen every tick to see what I mean. It’s not all that stable.
I’m in the planning phase of adding music to my game also. The best way I can see to sync tracks is to have them all start at the same time and fade them in and out. This might be a bit of a weight on the processing and memory, but I’m hoping it will be OK. Portal 2 has several tracks all playing at the same time, that just fade in and out.
Please let us know if you do manage to work something else out! Audio in UE4 still seems pretty primitive (as it was in UE3).