You can use the callback that you can pass in PlayCutscene so that you disable it when it finishes. Please take a look at the included scripts like CutsceneStarter for an example on how to use the callback.
Regarding MasterAudio, it is unfortunately not possible to do that, because Master Audio is a dynamic audio system and as such it is no possible to pre-determine length nor sample a specific audio clip since clips are determined on the fly.
Gavalakis
