A few of the recent revisions had intermittent audio streaming issues due to one of my pre-loaders screwing up caching, but that was adjusted and fixed before this version went out. A more likely culprit is that RPGMaker doesn't like where you've put it. Avoid deep directories (a folder in a folder in a folder...), the system drive (where you have windows), external drives, and anywhere with unusual permissions set. It may also be worth checking www\audio\bgm to confirm that the file in question is indeed there (the %20s are just spaces). Sometimes overzealous antivirus likes to munch various files right out of the install, (although usually those are dlls, not audio files) and it's possible your unzip process failed at some point (although unlikely).
If you've done a fresh unzip, in a "safe" location, you're sure the file is there and you're still getting the error (especially in a really reproducible way, like every time you try to start the gallery) let me know.