Music Editor - Video To MP3 is an app that allows you to extract the audio from your video files. It supports various video formats such as WAV, MP4, AVI, MP3, etc., and also has various audio formats such as AAC, M4A, WAV, etc. You can combine multiple clips and then convert them into a single audio file. 

The best part of this app is that it's free, you can use it without having to pay a penny. It also has a simple user interface, which makes it easy to use.