I took Casey's advice and emailed the US Copyright Office the following question:
"I wish to register a sound recording including the music copyright. The recording will go on sale late January or February. Should I mark it unpublished or published or should I wait until it is released to the public before registering the copyright and then mark it published?"
They replied:
"That is your choice to make. Works may be registered regardless if they are published or not. A work cannot be indicated as published until copies have been made available for sale and/or distribution to the public. You cannot provide a projected date of publication in the future, it has to be actual."
I had a further couple of questions which I got answered by the copyright office and thought I'd share them here as well for the benefit of others.
Q: If I send the recording to the distributor 2 months prior to it's scheduled release date is that period still classified as unpublished?
A: Yes, that is correct. Until copies are distributed or sold to the public the work is still unpublished.
Q: Is a copyright registered as unpublished still valid once the work becomes published or does one have to submit a new copyright registration once the work is published?
A: Yes, the copyright for an unpublished work would still be valid after the work becomes published. Also, once the work is actually published, you have the option to also register the published version although it is not required. However, if you choose not to register the published work, you should submit 2 copies of the published work as a mandatory deposit. When submitting a mandatory deposit, there is no application or fee involved (because you are not registering the published version), you simply submit 2 copies of the published work. If you like, you may include a transmittal letter indicating you are submitting a mandatory deposit. For additional information and the mailing address for sending mandatory deposits, see Circular 7d at http://www.copyright.gov/circs/circ7d.pdf .