Creative zen xtra 60GB mp3 player question...
First thing I'd do is compare the sound coming from your MP3 player to the sound of the same songs playing from your computer speakers, to eliminate any possiblility of the audio files being at fault (I realise you haven't altered them yourself but they could be faulty at source)
Then check the graphic equaliser in your player and see what the current setting is. Most audio devices these days come with a range of presets and these often include a karaoke setting so I'd check thats not been toggled by mistake.
Then have a play with your EQ settings to see if you can bring the vocals back up in the mix, vocals tend to lie in the 1-2kHz range of the audio spectrum so give it a boost in this region. You might also want to try cutting the bass end, too much bass energy in a mix can steal your headroom and lead to parts dropping out unpredictably like this. N.B. if your player only offers EQ presets rather than a proper hands on EQ then look for presets with names like Mid boost or Bass cut, even a general POP preset should be OK, just avoid things like Dance, Hiphop and the like.