my aim is simple: Just play a mp3 file using Mplayer, and want to use Jack and Jack_capture to record the sound that I have heard in my speaker. I know this must can be done by using Jack and Jack_capture, but I really have no idea about how to use them tegother. I have done as the "man jackd" said:

#jackd -d alsa -C hw:0,0

but "**** alsa_pcm: xrun of at least" will be given.
sometimes when I try some other parameters(I'm really crazy with the parameters!), mplayer can not be used, said sound card is busy.

I think the port must be set, but I don't know how to do that even after googled much.

help me,please!