1. If browser page contains the keyword(eg."YES") , then play song A or alarm.
2. If the page NOT contains the keyword , then play song B or keep silent is also ok.
Have you figured out how to play the selected files yet? If so, then you just need to read the page you want, using Net > HTTP Get. Then follow nanobrain's suggestion:
If [%HTTPD ~ *yes*]
If %HTTPD ~ *yes*
If you export your task's description here maybe someone can see what's wrong.
Also, for debugging you should either flash %HTTPD or set it to a new global variable, ie: %Httpd, in order to check its contents.