Judging by the OS market share numbers (Windows having 90%+ market share), I would think that the majority of the people using a personal computer today would stick with the default, which is Windows Media Player.
Now, if you want to get into more detail, you could look at different niches and aim a plugin at certain players. I would think that people who really enjoy paying for their music would use iTunes, but there is also the Zune Market & software.
Often you can find data on this in certain places for different niches:
I've started a survey with your question.
It's not over yet, but you already can get good impression: http://askyourtargetmarket.com/branched_surveys/49506/statistic/charts?share=4afd5f731732085f0739265d6ef1ff9f