I have no experience directly with this game, but...
While waiting for the official solo campaign, there are many free audio programs you can download that will let you manipulate the speed of audio playback. Before I heard of the official solo campaign (which was an awesome treat), I planned to play solo controlling 3-4 characters and simply play the soundtrack at something like 90% speed. It really depends on how good you are at managing zombie placements and things by yourself, so tweak it to your skill level.
Now that is a great idea! Let me know when you try it and at what speed you find gives enough difficulty