The first place to start would be Basics of Video Sound, Des Lyver, Focal Press. This small book will give you the basics.
Next, I really like Using Audition, by Ron Dabbs, Digital Video Experts Series CMP Books. I know that this is for Adobe Audition, a totally separate audio-editing program, but Dabbs goes into great detail on the various Effects, their use and on the settings. Obviously, all of the Effects (especially the myriad VST plug-ins) in Audition do not appear in PrE, but for the ones that do, you'll get a complete education on their use and what to expect from the settings.
Steve Grisetti has several Muvipix books, plus many articles and tutorials, that cover many of the Audio aspects of PrE, and goes into detail on the uses. His coverage of the Audio Effects WILL apply directly to PrE.
Now, for what you wish to do, I'd first get a SFX clip of crowd noises. I have accumulated dozens of these over the years. Some have come in SFX libraries, some are free from the Internet and some came from footage that I shot. They differ by the size of the audience, the ambiance of the rooms, the sizes of the rooms, and the type of crowd reactions, etc. Here are some likely sources:Some of those are for music, but most also have some sort of SFX catalog as well.Then, I'd do a duplicate of your narrator's Audio, placing it on another Audio Track. Add a hint of Delay and some Reverb to it, and drop it's Level by about -2 to -3dB below your original Clip's Level. Experiment with Pitch Shifter (go very lightly and listen closely), and also the Highpass, as you say that his voice is "too crisp." Then add Dynamics, and check Compress. Work with the Threshold and the Ratio, then the Attack. Careful listening will be your best determinant here. Then, move to Expander to "Gate" the Audio a bit. Again, careful listening will be essential.I recommend a good set of noise-cancelling headphones, and patience.There are also tons of VST's that one can download. I do not know how well, or how poorly, PrE will work with any of those, but there are some like Grunge, that will "muddy" up your sound. I know that Audition will load and work with many, but I do not use any of these in PrE, so cannot comment. I think that Audacity, a freeware audio-editing program will allow the use of some. There are many sites with both free and shareware VST's, so you'll need to look through a bunch. Then, there is the potential problem of PrE not being able to load some of those. This would be a good PLACE to start.Magix Music Studio also has a ton of VST's built-in. Might be worth a look.Good luck,Hunt