Hi elbon,
Have you tried previewing the voice to see what it sounds like through your own speakers/headphones? If not, can you give that a go and let me know if it still sounds muffled?
Another thing you might try is to record your preview using an audio recording program like RecordPad or WavePad. To do this, first run Voxal and then run your chosen audio recorder. Make sure the voice you want to preview is selected in Voxal and then start a recording in your audio recorder. Voxal should modify the recording as it is made. Once you are finished you can listen back to the recording to see what it sounds like.
Regards,
Ben