Hello,

I have noticed that some downloaded MP3 files have poor high frequencies even at high bitrates while other MP3 files at 128kbps have better high frequencies. It's curious but I don't know why.

Well, is there any way (software, etc.) that can improve the high frequencies of MP3 files?
Of course, the MP3->WAV->MP3 process is mandatory, but it's not a problem to me.

Thanks.

Most likely a lowpass filter was applied during encoding which effectively removed those high frequencies, so no, I don't think there is anything you can do to improve them significantly.

I am pretty sure there's no way you can add the high frequencies to the existing mp3s. Of course, if you still have the source (i.e. the cd or lossless) you can make new better mp3s

You should however be able to boost the existing high frequencies with an equalizer or similar. I know it's not the same thing but it might somewhat emulate what you're looking for.

Just turn off the lowpass filter.

There are some plugins which perform some kind of spectral band replication, adding fake high frequency signals calculated based on the lower frequencies. DFX does this (or at least it claims to do so).
Of course this only works to a certain extent. You cannot "rescue" a 11-kHz-lowpassed file with it, but on some material it makes quite a difference.

The most stupid advice in a while. If you dont know how mp3-encoding works, then dont act as if you know what you're talking about.

If you're looking to degrade the quality of your files then yes, this process is mandatory.

Since decoding and re-encoding isn't a problem, you could try using a harmonic exciter, which will build high frequency harmonics based upon the existing sonics. There are a number of free VST harmonic exciters that can be used with any VST-capable audio program.