audio recording file to be cleaned up (remove background noise, enhance voice etc)

Hi, I have an audio file that contains approx 6 hours of voice recording that was taken on a handheld voice recorder at a conference. This file has a lot of background noise and portions of the file it is very hard to hear the voices. I need to have this file enhanced by a sound engineer to remove all the background noise and enhance the voices so that they can be heard clearly. Experience with this type of work is essential, as is the ability to start straight away. Thanks