Mix multiple sound samples into one file.

Hello, I have a beat maker application that lets the user create a beat from different sounds. There are 10 buttons on the UI and when the user presses a button it plays a corresponding sound. Also under every button there are spinners that lets the user pick what sound they want playing from that button. I have a recording function in the application that lets the user record what they have played. It does this by adding the sounds to an arraylist and adds the corresponding times between each button click to an arraylist as well. Now what I want is a function in the application that lets the user record the beat they just created and play it back and if they would like to save it to their phones, they can. If they don't want to save it they can delete it and record another one. Attached is a picture of the application for some reference. Thanks.