Using one of this solutions will be more efficient. It saves you time, because there's **no more need to manually sync** the presentation and the voiceover. 


Free solution:

* Keynote can record a running presentation with the mic-audio. 
* QuickTime X has also a screenrecord-feature incl. audio. 

Lowcost solution:

* Specialized screenrecording software like ScreenFlow, Camtasia, Screenflick, Screenium, etc. 

Pro solution:

* Capture hardware like Ninja Atomos, HyperDeck Shuttle, Intensity Pro, Aja IO, etc. can record directly with ProRes-codec.


A professional solution with capture hardware can record directly in Final Cut Pro X (and other NLE) native codecs/formats like ProRes422. This saves much of time. The picture and the audio will be always in sync, and in addition you'll get a recording with smtpe-conform timecode. – There are even more advantages, but they will be more important if you record live presentations. 


P.S.: I worked for several years with dedicated screenrecording software to record live presentations. I've tested a lot of them and there is a lot unreliable software out there. Today i use capture hardware.