What you have is very close. To index into an array of durations, you could use:
!Index of: {'mySample.wav' fileNamesOfSameTypeInSameFolder collect: [ :wav | wav fileDuration]}
The message fileNamesOfSameTypeInSameFolder returns an Array of file names. Then you are sending the message collect: to that array and, in the block, you are gathering up the durations of all the filenames in that Array (and returning it as a new Array of durations). Then you can index into that Array. Note that !Index should have the range [0, <nbr wave files - 1]