Hi...
The
quality ( or lack thereof
) of the .mp3 files you have is determined solely by the manner in which they were 'ripped', i.e. converted from the 'original' format to the compressed format (in this case .mp3...)
It is not something you can alter to the 'better' after the compressed file has been created.
Most programs that contain applications that permit you to 'rip' from the original format will include many options for setting the resultant 'quality' of the compressed format 'conversion'.
If you are running Windows, try
CDex 1.51. You can find the download page link for it in my 'sig' below.
It is an excellent stand-alone program that performs conversions, with many options and excellent results.
Price is good, too !
There are many ways to do the ripping/converting...
Many of the programs available are not very good. CDex 1.51 is very good.