When why play pcm, quickly sometimes is sometimes slow ? 

 
The value you write to $4010 determines DPCM playback speed. What are you writing there? What sample rate is your original file? Did you apply any oversampling?
 
I am writing AIR MV  the  plan to make the background music with pcm。
Here is pcm and nes.
http://www.uushare.com/user/hd380000/file/1594145