First, anything coming in cia the internet (YouTube, Pandora, etc., is subject to buffering (waiting for more content to come in via the internet, because the path between you and the server is busy).
That said, if it's happening with your music player playing music that's located on your phone, it would be because your phone is malfunctioning? How? Any number of ways? It's a phone that doesn't have the power to do all that you're asking it to do? It's running a really old version of Android, but new apps? Something in it is defective? You haven't given us anything to go on except "it doesn't work", and we can't guess at why. You'll have to do a little diagnosis yourself, or bring it into a repair shop.