Imagine that you have some instrumental music files in your collection. You have a hidden desire to become a playback singer and you wish to sing these songs with the background music. What will you do? Well, the simplest thing to do is to put on headphones and play any instrumental music. Then sing along and record your vocal. OK, what's next? You need to mix the instrumental music and your own vocal together to get what you want!
Let's see how to mix two audio tracks together. The interleave is a GStreamer plugin that facilitates mixing of two audio tracks. It merges multiple mono channel input audios into a single audio stream in a non-contiguous fashion. This plugin should be available in your default GStreamer installation.