According to the developer of the mod the sounds in Minecraft PE is already a coded feature. But since the size of songs are too big they haven’t been added as an available feature yet. But if we remember it correctly it has been mentioned by Mojang that it will be a likely addon in the future.
By using this mod you can access the Minecraft songs (which are the same songs available for the PC version of Minecraft). There are tens of different songs which will load both on the start screen and in your Minecraft worlds.
To give you an idea how this works in-game watch the video down below. Sorry about the quality! It’s the best I could do with the resources I have.
It’s a little bit different compared to other mods as it’s in fact installed as a texture pack. It works on any version for Minecraft Pocket Edition.