Music from C64 games can be used, yes. Its impossible to contact the companies that made the music because everything related to C64 kind of doesn't exist anymore. Hench the fact that there's a huge community around remixing C64 tunes.
That's why it's tolerated, but those C64 tunes are still under copyright.
To quote :
http://www.copyright.gov/help/faq/faq-duration.html :
"As a general rule, for works created after January 1, 1978, copyright protection lasts for the life of the author plus an additional 70 years. For an anonymous work, a pseudonymous work, or a work made for hire, the copyright endures for a term of 95 years from the year of its first publication or a term of 120 years from the year of its creation, whichever expires first."
The C64 was released in 82, so all C64 tunes were made after this law, and therefore are bound by it.
That means that the earliest the C64 music copyrights will expire is 2052 (assuming someone wrote a tune on a C64 just as it came out, then died immediately after).
So if you really want to be safe, don't use anything made by anyone else in the past century. In the mean time, let's get things moving, because copyright might be intended to protect the artist, but it's killing derivative work, and is obviously stupid, because no-one can argue that 70 years after the death of an author the piece still has economical value (at least in the general case, where all the money is made in the first five years. Waiting five years to make a remix isn't that bad after all)