I think somebody should provide some background information:
Music on the C64 was created by writing a program, which controlled the SID (Sound Interface Device) Chip.
Most musicians on the C64 were composers, arrangers and programmers at once, many wrote their own
music playing routine. Music was sometimes written for games, sometimes for demos and sometimes
standalone. Sometimes this music was a cover.
Often people likes the music from a game so much, that they ripped it, usually by extracting just the part
from the memory, which contained the music playing routine and the music data. This part is usually
enough to play the music on a C64, if you know 1. where in the memory it should be, 2. at which
address the music initialization should be called and 3. at which address the music player routine
should be called at regular intervals (to play the next note, if required).
A SID File contains mainly this part from the memory and some metadata, especially the 3
aforementioned addresses, the title (for games the name of the game), the musician,
release information (who released it and when, for games the company) and the number of tunes.
It is sometimes possible to play different pieces of music or soundeffects by giving a
different tune number to the music initialization part of the player.
This also means, most times there is just a title for the whole thing, not for the individual
tunes. Of course sometimes it is known, which tune was played for which part of a game,
e.g. title music, in-game music, game over music etc. but this information is not contained
in a SID File. The title music is not necessarily tune 1.
A SID File can be played on other computers with a program (e.g. playsid or sidplay),
which emulates all relevant parts of the C64, mainly the CPU and the SID.
There is a big collection of nearly all music, which was ever created for the C64,
the High Voltage Sid Collection (HVSC) at http://hvsc.c64.org
The collection also includes a file, called the Sid Tune Information List (STIL),
which provides some background information about some of the tunes,
e.g. whether some other work was covered by it.
The Stone Oakvalley’s Authentic SID Collection (SOASC) at http://www.6581-8580.com
is a collection of MP3 recordings of all tunes from all of these files on a real C64.
In theory, it should sound nearly the same when the chip is emulated, in practice
there were multiple revisions of the SID chip with little differences and even
multiple SID chips of the same revision had different filter responses (for this reason,
C64 musicians, who wanted their music to be the same on each C64, seldom used filters).
The collection contains each tune played on 3 different revisions of the SID chip.
When a tune was meant to be played in an endless loop on the C64, the
recording ends when the tune would re-start at the beginning.
Some of this music was also covered by other people, sometimes with “real” instruments.
A lot of these covers are available at http://remix.kwed.org (they are called “remixes”,
but only a few of them are based on original recordings, so they are usually no remixes
in the MusicBrainz-meaning of the word).