I was asked to do the same thing by a friend of mine - it was a recording that I wrote 15 years ago. I used Sonic Foundry Sound Forge XP Studio 5. I recorded it into Sound Forge set the all around level once it was recorded and voila... Keep in mind that the digital recording wasn't what you would call perfect... you can still hear the hiss of the tape before the song starts. But once it is playing the music hides it.

Bobby

https://soundcloud.com/bobby-t-394727974