Re: MP3 artifacts
^^^ Correct. Those aliased frequencies are baked into the audio now, so there's no way to remove them except by re-encoding.
This is probably not relevant, but just in case...make sure it's not the playback system that's doing the aliasing. Many years ago I bought my first MP3 player, with the idea that I'd listen to my mixes while out on walks. It was a cheap device from Office Depot, and it sounded just awful. I'd had no prior experience with MP3 encoding, so at first I thought I must be doing something wrong. But the artifacts weren't there when I played the files on my computer, leading to the epiphany that not all decoders are created equal.
All else is in doubt, so this is the truth I cling to. My Stuff