If you use the TimeIndexForFileName Sound with a SampleWithTimeIndex (both set to the same sample file), then the TimeIndexForFIleName does read the loop markers when its loop mode is set to "Markers".
If you set it's ForwardBackward parameter to true (greater than zero), then it will bounce forward and backward through the sample just as the sample editor does.
If you want to make a Sound that starts looping with the loop markers in the sample's header but then lets you move them you could do that by extracting the markers with Smalltalk and putting them into the loop parameters of a sample player added to an !EventValue so you can change them at run-time.
Check out this question from Cristian and its answer to see how to extract the loop markers with Smalltalk.
This is what the looping parameters in the TimeIndexForFileName could look like to extract the looping markers but allow you to move them:
Note that you have to set the LoopMode to "Live" for this to work.
Here's an example Sound that does this.