Here is a speaker phone tweak that is supposed to increase the volume. I have not tried it because i don't use speaker enough to notice. I find my Telus speaker works fine but for those of you that would like it louder try this. I found it over at XDA so if you have any problems i will try my best to help but you may have to refer to the XDA site.
Please, before you replace the windows file with the extracted zip file, make a copy of the original and save it somewhere so if it doesn't work you can always put the original back.
enjoy!
Code
http://forum.xda-developers.com/showthread.php?p=3996585&highlight=volume#post3996585