Question about music which is no longer commercially available

I remember reading in a magazine a while ago about music blog's which have like songs and in some cases albums by singers/groups which are 'out of print' and not available to buy commercially which they'd put on their blogs for people to listen too. I was wondering, is music which is no longer available to buy in the public domain if the record label which originally released the album has gone out of business & if no one owns the copyrights to the songs and also if no other company has the rights to re-issue the music?. Anyone know anything more about this?, as I think labels like Hip-O-Select could re-issue alot more music onto CD if they acquired the rights to music which is out-of-print.