Look into audio normalisation.. for playback purposes scanning/applying Replay Gain on the files should help a bit. Most audio playback software has support for that.
There is also EBU R 128, a slightly different type of loudness normalisation, it uses a different algorithm vs traditional Replay Gain. For my own usage I found it works better keeping the loudness at the same level when playing through a bunch of different audio tracks. No idea about VLC but the Strawberry application does support it so it could be worth a look if you want to try other audio playback software.