Exported Audio is Muffled (Low Volume)

I’m a newbie using audacity and I am trying to digitize my collection of cassettes and vinyl. I have followed the equipment setup from wiki-audacity and the music sounds fine coming out of my PC speakers, but when I export the music into MP3-files and play them on my mp3-player, the music sounds low and muffled. I have tried amplifying (through audacity) the music, but it doesn’t seem to help. I’m not sure where to start looking for the issue. Has anyone experienced this and what can I do to make it sound better.

The bitrate chosen for MP3 export could be responsible for a loss in sound quality on the mp3 version, use 256 or 320Kbps for high quality sound …