[+] play song even if not in playlist
[+] volume changer must be software, and work with all codecs (at mix level)
[+] volume changer must work even no song is playing
[-] eat minimum memory (dont hold all metainfo in memory, etc)
[+] seamless playback (no pauses between tracks)
    * still needs some more work in some codecs
[+] playlist must be superfast