Audacity is a free open source digital audio editor and recording computer software application, available for Windows, OS X, Linux and other operating systems. (Wikipedia). Audacity is another free software on the internet and is useful to record any audio from voices, songs, tracks and etc. User can easily edit the audio once it has been uploaded in the Audacity. (click this link to get auto-directed to the download page). You can merge and mix up audios to your liking and features that are provided. After you have done with your mixing, save it either as Audacity Project or as mp3 file.
I print screened the image as below.
The How To
Open Audacity, click open file(s) on top right corner, or drag the music files directly on the screen. Mix the audio as how you wanted and save it. If you wish to save it as an mp3 file, you have to click on Export file and add .mp3 to rename your file for example Audio1.mp3 and click ok.
Hover up to TASK > AUDIO TASK, to see my work on Audio editing/mixing.