After looking up what "spotify" is.
-Their Artist terms of use agreement, has you indemnify spotify and leaves all accountability on the uploader/artist. (so they are in the clear for playing the songs)
-A label is going to have a contract as well more than likely holding them harmless if they were not disclosed samples or what samples were used. And are these tracks being sold or just played?
-As THEGREAT stated above, until these songs are brought to the public eye and show serious money making potential, what would be the point of a never known artist from the 70' spending $500 an hour on a lawyer to sue a bedroom label for the $100 in royalties they made off of spotify and iTunes.
At which point they still have to prove that the 4 second drum break that has been chopped/resampled/effects applied/tempo changed is their music.
-Also (probably not a lot out there), but work made before 1978 had copyright expirations of 50 years after the artist death (it looks like it has been changed to 70 Thanks Sonny Bono!). So I believe legally that would make any old artist music that died before 1948 up for grabs without any dispute.
Good Link for Copyright after death:
http://www.arsny.com/copyright-basics/(I'm not a lawyer, so anything written above is my
understanding of the legal side, and not to be taken as legal fact or advise) <--- See just covered my a**.