I think it is rather interesting how many of the 'engineer' posters in this thread seem to have some bizarre belief that they have some special ownership of the artists intellectual property just because they have tweaked a knob or moved a slider. If a recording studio is hired by an artist to record, they are nothing more than a contractor. A contractor who has absolutely no right to any of the work produced. In fact, keeping copies of the recorded work after the contract is completed without the express permission of the artist is illegal and represents an interesting liability (what happens if copies escape?) Any special ju-ju that the engineer uses to create the recording actually become the property of the artist unless there is a special agreement in advance otherwise. If the engineer re-uses that same 'special' sound without the artist's permission it could be grounds for a pretty solid lawsuit against the studio. While this does not give the artist ownership of the intermediary products of the recording, it clearly makes re-using them a liability for the engineer unless there is some clear intent of ownership drawn up in the agreement.
OK you state all this as a fact, but I would be interested in how you determine that a contractor has no right to any work he produces.
Example: Crabgrass Symphony Society commissions a work to be played on the opening day of the season. Since they pay the composer, but do not want to be liable for his benefits social security tax, witholding etc. they execute a contract saying that he is an independent contractor, give him the money and issue a W-9. The contract only requires him to deliver the composition, and is mute on the ownership or transfer of copyright, and does not state that it is a work for hire. Under black letter copyright law he retains ownership of the composition regardless of the fact that he is a contractor.
Being a contractor per se does not transfer or invalidate ownership of the work product. If the contract he signs states that ownership is transferred unambiguously, or states that it is a "work for hire," (and it meets the statutory requirements) then there is a basis to believe that. Absent that clarification, then you would have to depend on what was "understood" between the contractor and his employer, and argue in court that an unwritten contract did include such a transfer of rights. You could use the standard industry practice to buttress your understanding of the situation. If no recording/mixing/mastering engineer has ever asserted ownership to his rights in the phonorecord that is his creation under law, then you could say that his belief that he has such ownership was so bizarre that it could not have been the understanding that makes up the unwritten contract. But that is far from a fact--at least until a court decides it.
The engineer clearly can not claim any rights in the original composition or the performance that he processes. But a recording encompasses two distinct rights. One is for the composition, and the second is for the phonorecord, which is the actual captured sound. If the engineer just turns on the tape recorder, it is difficult to see how he has produced anything that he can own the rights to, although arguably he has. If he does major mixing, selecting and discarding tracks it may in effect rise to the level of an arrangement gaining its own composition rights. If he uses all of the tracks as given but adds effects levels etc. it pretty clearly has the effect of being an original creation recorded in tangible form, and absent some other consideration it is his property under copyright.
If you are engaging the services of someone who is going to give you back a phonorecord, it would be wise to insist on a contract stating that at least the copyright to the final product (phonorecord) you receive is your property. The most reliable way to do that is to be sure that the contract meets "work for hire" requirements, in which case the engineer truly is, "a contractor who has absolutely no right to any of the work produced." Otherwise is not inconceivable (although it is probably pretty unlikely) that the engineer could assert rights arising from his own work on the project. The law is no different for an informal garage based operation than from a major recording company. Those offering their services as engineers, should consider an explicit clause in their contracts transferring copyright in the final phonorecord to the client. I believe that is what you all intend to do, but you can not guarantee the actions of people who may inherit or obtain your rights in bankruptcy etc. with your own unwritten words or intentions.