The biggest mistake is taking plural vs. singular out of context.
EXP... mp3player and mp3players both work grammatically, the singular is probably stronger in this instance for a search because it catches both terms equally well. On the other hand, are you selling ONE mp3player? While the term might be stronger in the singular form the plural (as JayJay pointed out) pulls more searches simply because it's more natural to use the plural. So the point may be moot simply because the vast majority of searches will be for players vs. player. I can also make a case that the singular would pull for both terms which would give you all 5500 or so searches.
If you owned sonymp3players that would be incredibly strong for a term that garners a ton of niche search pull. The problem with your URL is that "quality" is so generic. It could just as easily be great, fantastic, good, awesome... etc. You have to take into account the amount of search pull quality+mp3player(s) will have. Subtle changes to a given keyword phrase makes huge differences in pull.
Hope this helps a bit
Cheers