Music is different from sounds in Android. While sound files are usually short and they are loaded into the memory, music files tend to be quite large and they are streamed from the storage and only a part of the music file is currently in memory. Also, only one music file can play at a single time.
The other problem with music files being quite large is that they take up valuable space. The APK archive is limited to 50 MB. While simple games tend to be much smaller, in resource-rich games the limit can be reached quite easily.
Up to two expansion files, 2 GB each, can be added to the game. You can learn more about expansion files at http://developer.android.com/google/play/expansion-files.html.
We have already loaded the music file ...