Low vocal Creative Zen xl

Turn gain CCW. plug microphone in, mute the channel. turn gain CW until peak LED flashes (while singing loud!) and turn gain back a little bit.
Take the channel fader down, unmute the channel. Put masters at 0dB (near maximum) and adjust the loudness with the channel fader to what you like.
EQ: it always depends on your location. Keep it flat. You might like to reduce at about 200-300 Hz. Cut the low end (Hi Pass Filter = on) or if no HiPass reduce "bass" for about 6...10dB.
ok?

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.

You MAY be overdriving the sound card. Near flat should work for vocal, but setting the levels throughout the mixer is important as well as the sound card. You can use the LOW CUT to reduce hum if you have noisy system. Describing balancing the levels is more than I can do here. There are many good videos about mixer setup on YouTube.com. You cannot "fix" problems created by improper levels and noise injected into the system using the EQ.
What is important is to see that all gain controls and faders are around mid range to 70% when operating normally. You should be using BALANCED lines everywhere possible either XLR or TRS cables. ALL interconnected equipment should be powered from the same receptacle or power source INCLUDING all amps and powered speakers. Often users use unbalanced lines from mixers to sound boards and pick up noise or they have the levels set wrong between the mixer and sound board resulting in clipping and distortion or poor signal-to-noise ratio.

It sounds like the guitar level input to the unit is too low.

NOTE: With this product you MUST use an amplified or electric guitar that has a preamp within it.

If you have a straight guitar that does not have an amp in it, buy a preamp to first run the guitar through.

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Your hard drive might have broken during the process. To troubleshoot the hard drive and your Zen in general, please click here

you blew up the audio output for one side,either send it in for repair or by a used as is one from ebay and swap module.

Sounds like maybe the head jack is loose.

http://www.creative.com/products/mp3/MP3PlayerRecoveryTool/

On some creative mp3 players like my 20g sleek photo, the problem stems from poor design on the solder joint inside the unit. I did a search and found a tutorial with pics on how to take it apart and resolder it. It works now but cosmetically it's not 100%. If it's still under warranty, I think for $20 bucks you can send it back and they'll fix it....goodluck!