It's exactly as easy to decrease VLC's audio level back to normal as it is to increase it.
Anyway if you don't like it remembering the last audio level, go to Tools > Preferences > Audio, check the "Always reset audio start level to" box, and set it to 100%. Voila!
Anyway if you don't like it remembering the last audio level, go to Tools > Preferences > Audio, check the "Always reset audio start level to" box, and set it to 100%. Voila!