DE2's example--DE2_i2sound 'problem

Hi,now I am finding a problem in the DE2 example--DE2_i2sound.

In the original sw,in the CLOCK_500.v,

ROM[2]= 16'h0838;ROM[3]= 16'h1000;

I want to select the DAC,the line input as sound input,and set the sample frequence to 8khz,so I modify it as below:

ROM[2]= 16'h0810;ROM[3]= 16'h100c;

after I compile it ,I download the .sof to the fpga directly,I can hear the sound from the line,it runs well!

but when I download the .pof to the fpga,and after I reset the fpga,

I can't hear the sound ,but the frequence such as ADCLRCK,DACLRCK,BCLK,XCK are all ok.

so where is the problem?

thank you.