hmm.. Try playing them with another Audio/media player like Winamp or Windows Media Player. XoloX uses whichever windows default you have for the file type, so make something else your default audio player. ( real player in my opinion sucks).

When possible get files with the highest score - usually it's good quality and legit.
Also the higher the bitrate , the better the audio quality will be. Try to shoot or the files at least 192kbps , 128 will suffice.

"Q. What is the Bitrate of an audio file and what difference does it make?
A. Bitrate is a measure of digital audio sound quality. A Bitrate is measured in kilobytes per second. Common Bit rate's you will find are 128kpbs, 192kpbs, and 256kpbs. The higher the Bitrate the better the audio fidelity is."


p.s.
I prefer winamp for my audio - http://www.winamp.com/download/

In future builds you will be able to enqueue files from within xolox ( or any default player as you can do now ).