Audio editing is not possible in the application primarily because it doesn't have this feature available. You will need another application that was developed exactly for this purpose. One that can work with waveform for better handling of the audio stream. The application that you have to use is called Audacity and provides all the necessary tools to mix and edit the audio streams of the loaded files.
You can download it through the following link: http://audacity.software.informer.com/
There are also plug-ins that can be added to the application.