Well, I don't know about it being a mockingbird because the original poster didn't mention the song constantly changing (like a mockingbird's would).
I'm going to throw out Whip-poor-will and Common Poorwill just to see if those might be it. I'm thinking that if it isn't a mockingbird then probably the Common Poorwill as I would describe there song as "sweet". The song of the AZ Whip-poor-will is ruff and not like that of the Whip-poor-wills back east, though it still might be that.
You can listen to the Common Poorwill here and the Whip-poor-will here. Just imagine the song of the Whip-poor-will being slower and more burry and you basically have the song of an AZ Whip-poor-will.
Hope this helps!
featherbrain
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