So, after a heavy session of AC/DC mp3s, you return to work until-JEEZE! WHAT THE HELL WAS THAT? That was an ordinary beep indicating some error-only played at a volume so high at first you think you're being shot at.
MP3 music and Windows event sounds simply can't get along on the same PC-unless you have Wavosaur, a free sound editor that includes a batch operation to lower the volume of all the wave files in a folder by 6dB. Doesn't sound like much at first. You may have to run your event waves through it a couple of times. (Do make backups in case you overdo it.) Wavosaur is so simple to use and works so quickly, in minutes you'll have a respectably decorous set of sounds you could take to a Zamfir concert.
And did I mention that Wavosaur edits, processes, and records sounds, .wav files, and MP3 files? Wavosaur has all you need to edit audio, produce music loops, analyze, record, and batch-convert.