The best alternatives to SongBox Player are Jukebox, BoxPlayer, and Diffuse. If these 3 options don't work for you, we've listed a few more alternatives below.
Diffuse is a decentralized music player consisting out of two main parts. One part is the music and the other is your data (eg. playlists), both of which are in locations of your choice. Meaning that there’s no central server for Diffuse, all of the processing happens on your device and all the data is in your control.
CloudTunes is an open source webapp that with a unified UI for music stored in the cloud (YouTube, DropBox, etc.) and integrates with Last.fm, Facebook, and Musicbrainz for metadata, discovery, and social experience. It's similar to Spotify, but instead of local tracks and the fixed catalog, CloudTunes uses your files and YouTube music videos.