Subsonic is both an app and a cross-platform FOSS media server that's capable of indexing very large media collections. The server can transcode if necessary so that the app can play files that your device may not normally support. There is a demo server provided for giving it a try. In order to use this app with a Subsonic server beyond a trial period, a key must be obtained by making a donation. (A key isn't required to use the <a href="https://github.com/Mach5/supersonic">Supersonic</a> variant.)