"If the artist paid the studio up front for recording services, the artist owns the master.
If the studio or record company financed the recording, then they own the master."
It can get a little more complicated than that. Without a contract, ownership of the Master Recording can become a very sticky and lawfully expensive problem.
The argument of ownership of Master Recording is simplified to "who owns the copyright." The law in the U.S states that once the lyrical and musical content is created, the copyright belongs to those parties of creation. Submitting and paying for that copyright essentially gives you more protection under the law, but it is already statutorily implied once it is fixed to a tangible medium.
Even if an artist has not paid his studio fees (if there is no Record Label or other exclusive or non-exclusive contract in place) the Studio still cannot release the song for commercial or promotional gain without written consent from all persons entitled to ownership of the copyright. (since part of the intellectual property still belongs to the creators of the music.) Furthermore, they cannot destroy that master recording either. (it is unlawful to destroy someone else's intellectual property). I have seen the argument used that since Congress states in the copyright act that "capturing and electronically processing the sounds and compiling and editing them to make the final sound recording "may be enough for a claim of authorship", studios that have not been compensated for their services can manipulate the Master Recording on a non-exclusive basis (once the other copyright owners are also compensated), it is still a very grey area. It entails the law stating "may be enough for a claim of authorship." They would have to prove that they added to the song in a "significant way" and also, why they are due authorship/ownership. This can be very hard to prove without a written contract, and courts have found for and against this argument. It further complicates the situation because as a point of law, recording , mixing & mastering cannot be said to create a "tangible work" that is "significant." One would have to prove that if this song was recorded by, mixed by and mastered by the Creators of the musical content, that it would not be the same "song" capable of the same level of "success". Viral marketing only hurts this process, since we have seen many amateur recordings done on YouTube give tons of people overnight success. Furthermore, the Studio would have to secure a Mechanical license from the band/artist/composer before being able to Publish the song.
The real life scenario is that the authors of the music (composers, writers and sometimes the producer) own the Copyright unless there is a specific production or Label agreement in place which already states ownership as otherwise. Even if someone else has flipped the bill for the song to become a "Mastered Recording", that in itself is not enough to statutorily grant ownership. It is merely a loose (and unenforceable in most cases) rule of thumb.