I can’t recall what thread I was reading when I read a comment about old stereo recordings having complete separation of instruments and/or vocals and how annoying it is to listen to on headphones. I agree it is annoying and basically is attributable to studios tinkering around with “new technology”. Regardless, if you find this annoying as well there is a program that may help you. It’s called Audacity and is free. I has been listed in PC Mags to 101 Freebies in 2007 & 2008. I know it will allow you to convert the mp3 to mono and I think you can copy one channel and add it to the other which might produce a stereo type effect. The program will do a lot of things. The most common thing I use it for is to shorten the length of a track. Suppose a track is 12 minutes but you would like a shorter version for some reason. You call up the mp3 file, find you a good place to fade out the track. Highlight about 5 seconds or so, select Fade Out from tools and it fades the volume out. Then you highlight the remaining part of the song and select Cut. Now you have a shorter version of the track you can export to a new mp3 file. It can do things like change the pitch, increase the volume. A friend of mine uses it quite frequently in his production of videos. The volume increaser I have found useful on recordings I have made of lectures and such.
Like, I said, it’s free and works pretty well. When you install the program it will ask if you plan to export to MP3 and give you a lame codec. That’s the only quirky thing to me about the program. MP3 is the most common audio format and would think it would default to that when you save but you have to select Export to MP3.