To answer your question I downloaded the application and here should be an answer to your question. You need to change the level of volume, so you can level all you mp3's again on a higher volume. To do so, follow the following instructions:
When in MediaMonkey:
From the main menu, click Tools > Options > Player > Volume Leveling.
On the configuration panel set:
upgrade the volume of leveling to something you like, for example 100 db. Now here comes the important step, unmark the ''Only analyze Tracks with unknown leveling adjustment'' box. This will volume level all your tracks again. In the same menu you can also change the playback volume (upgrade this too, if you think it's too soft after leveling at 100db) If you have done this, things should work properly again. When not:
There are various programs around to make your mp3s louder, for examle there is MPTrim:
But this application only allows you to do this with one mp3 at a time, so this can be a very long job.
Since you already tried changing the volume slider without any ffect I think the problem is really with your mp3s. I hope the above was helpful enough to solve the problem