I have three versions of Movie Maker installed on my Windows 8 laptop and usually edit in MM 6.0. In that case, I've taken to using KMPlayer to capture (that's the menu it's under) the audio so I can then boost the volume in Audacity, export it from there as an MP3 (you can also export wavs and several other formats) and then finally import that version into Movie Maker. Tedious, but it works, and sometimes there's no other choice. Sometimes just capturing the audio with KM Player is enough, though, and then it can just be imported into movie maker.
Movie Maker 2.6 seems to play the existing audio louder than MM6.0 or Live does. Try to edit the same file with all three versions and 2.6 is the only one that starts with an option to make the audio even louder. The slider on the other two versions is already at the max, so if your audio is too low, there's no way to increase it there.