If my video contains a musical recording and the owner of the recording master rights authorizes YouTube to place ads on the video, do I have the right / capability to prevent ads from being displayed?
I assume you're talking about the music offerings from YouTube themselves - the ones they'll dub in for you automagically. If this is the case, no, you do not have the ability to block ads or monetize your own videos. This is how the providers of the music get paid for their offering.
If you'd like to dub in music and either have no ads or monetize your own content, you'll need to find a track that is explicitly licensed under creative commons (without a non-commercial clause) or similar license. YouTube will also ask that you provide a link to the location the file was downloaded including its licensing information. The audio will have to be dubbed in before upload and YouTube may still reject your video if the licensing isn't 100% kosher.