In order to post an audio file (like a song for example) to your feed, there are 6 steps you need to take
Once you open the app, tap "New Post" then select "Music"
Make sure to review and accept the Content Guidelines
Tap "Upload File", Select the file you want to upload and then tap "Continue"
Tap "Choose Cover" to select your audio file cover picture and then tap the image your want to select
Add a title and a caption
Tap post and that's it!
Note - If you want access to the music feature you must first be approved to post audio files on the app. You can do so by filling in the application you land once you click on "Music" (see step 1).