iOS 8 Video Editing App That Lets You Hear The Audio While Trimming?
There're many great video editing apps in the AppStore. Unfortunately, I'm not allowed to name a specific app according to the StackExchange policy (my previous answer was banned). Some of them play sound while you do video editing (trimming, applying video effects, etc.)

For sure smartphones now have enough computational power to do very complex video editing, they have GPU for even better performance. As a such kind of apps developer I admit that only a tiny fraction of professional desktop video editing software features couldn't be implemented on a smartphone. The major limitation is the screen size. It is freaking hard to make an intuitive user interface and a great user experience on such a small screen.