The http://www.kjaerhusaudio.com/classic-series.phpKjaerhus Classic Series[/link] are a series of free audio plugins that are just incredible. Get them all, but the one you want for this application is the http://www.kjaerhusaudio.com/classic-master-limiter.phpClassic Master Limiter[/link].

If you want to spend money on something even better, the Kjaerhus Audio http://www.kjaerhusaudio.com/gold-series.phpGold Series[/link] gives you the same great audio quality, but more control. Of these, the http://www.kjaerhusaudio.com/gpp-1.phpGPP-1[/link] would be what you are looking for. I have this, but to be frank, the free one works just as well 90% of the time.

Another plugin of theirs that I just love and might be applicable to your situation is their vocal channel processor, the http://www.kjaerhusaudio.com/gac-1.phpGAC-1[/link]. This gives you everything you need to fix a vocal track. I use this mostly on sung vocals, but it can also be good for voice overs.

If you recorded the spoken part in a room with ambient noise, the free Classic Master Limiter will tame those occassional peaks and this will be as far as you really can go.

On the other hand, if you recorded the spoken part in a quiet room without ambient noise, the GAC-1 will let you work absolute magic.